Water changes for 90 gallon Oscar

David R said:

I know there is a lot of talk of this, and a lack of proper scientific evidence too, but obviously it does happen to some degree (thus why discus breeders do regular huge water changes on heavily stocked fry tanks). For a single small fish in a 90g tank I find it hard to imagine it would be an issue though. I know people say "more is better" in terms of water changes, so why not change 90% of your tank water twice daily? That's gotta be better than 75% every few days eh. You have to draw the line somewhere...

OP; your oscar isn't going to die if you can't/don't do 75% water changes every few days. If you want to do that (or more) then go for it, but I'm confident that you'll be able to keep it happy and healthy in that sized tank with a more conservative water change regime. At the end of the day it's your fish so you decide. Nitrate levels aren't the only reason to change water, but they can be used as an indication you're changing enough. If you keep them in check I'd say there's a good chance you're sufficiently diluting anything else too.

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...Nicely said; there are a lot of test kits out there but most only cover basic tests for things like PH, nitrite, nitrate, ammonia, etc. So using nitrate as a base to test the parameters of your water for all the things you cant see is a fairly solid way to get an idea on if you are doing enough water changes, etc. Also I agree on the "line drawing", more is better but this is a hobby not a job. IMO The only way to really tell if your doing the right amount of water changes is to test your water. If you are keeping your parameters in check then you're good, if not, then step it up.

On a bit of a sidetrack:

Without adding a page full of quotes, I'd like to throw my hat into the ring on the Oscar size conversation: I have only kept 3 Oscars over the course of somewhere between 6-7 years, so anyone here is welcome to take my opinion with a grain of salt: Although I believe that 12" is what most people would consider average size for an adult O, I do not believe it is the max for well kept home aquarium O's with adequate tank size, optimal water parameters, a good feeding schedule and quality diet. I have never been on an Oscar max size picture hunt online but I can tell you that I currently have a 6 year old male O around the 13" mark and have lost an O (she was about 5 years old) in the past that was 14"+ ...sorry no measuring tape pics. That being said, I also have a 3+ year old female that just cant seem to hit the 12" mark and I doubt she ever will.

-OP, to answer the original question in the thread:

I would expect to change between 35-50% water weekly while performing a thorough gravel vacuum, you may even do this twice weekly if you have the time, or if the water parameters indicate that it's nessecary. At least thats what I've been doing for the past few years with my O's and it seems to be working out fairly well.

This is just my opinion of course, I'm sure you'll find it differs from many others online. As stated before, if you keep your eye on water quality/parameters, perform adequate tank husbandry and keep a good feeding schedule; then you should be able to figure out what percentage of water should be changed out weekly. I personally shoot for less than 20ppm nitrate at the end of the week before my water change, if its higher than that before the change, then I just do more.

Good luck with your new O, make sure to post up some pics of your new tank and fish once you've got everything all set up!

A single healthy 14" oscar will weigh around 1.8 lbs and need 20 grams of food (90% dry weight) daily to equate to 2.5% net food to net weight. That's a bit above absolute minimum maintenance needs for adult tropical fish (which varies species to species), so it assumes a modestly active oscar, which I presume we all want. Assuming the food is 10% moisture and 50% protein, that adds 1.2 grams of ammonia daily. In 80 gallons of water, that is adding ~4ppm of nitrates daily.

One would need to remove ~60 gallons (75%) per week and replace it with pure 0 ppm water to keep the nitrates under 40 ppm over the long term.

Now.....a 12" oscar only weighs ~1.1 lbs. Let's assume it's fed less---2% net food to net weight---which is only 10 grams per day. In that case, one could do just 40% WC per week and stay under 40 ppm in nitrates.

So, there's really a dramatic difference between a 12" oscar fed barely at maintenance (10 grams daily) doing 40% WC a week versus a 14" oscar fed 20 grams daily doing 75% WC per week.

Needless to say, if the nitrates are high enough and/or the feeding low enough, the oscar may have a difficult reaching 14". So, it's plausible that many oscar owners have raised them to ~12" and not continued to increase WC and/or food at that point, such that the oscar never is able to grow further.

A 16" oscar would weigh in at ~2.7 lbs (50% heavier than a 14" oscar, 136% larger than a 12" oscar) and require that much more in food and WC.

Good to see someone actually crunch the numbers!

Obviously a bigger fish will consume more food, but would there be any reduction in appetite/consumption (possibly a stupid question given how oscars eat!) as the fish gets closer to its max size because of the slow-down in growth? A 6" fish will eat a lot less than a 12" fish in volume, but it will also be growing at around 1" per month where as a 12" would be more like 1" per 6-12 months, so would it eat more proportionally to it's size to sustain the more rapid growth?

David R said:

Good to see someone actually crunch the numbers!

Obviously a bigger fish will consume more food, but would there be any reduction in appetite/consumption (possibly a stupid question given how oscars eat!) as the fish gets closer to its max size because of the slow-down in growth? A 6" fish will eat a lot less than a 12" fish in volume, but it will also be growing at around 1" per month where as a 12" would be more like 1" per 6-12 months, so would it eat more proportionally to it's size to sustain the more rapid growth?

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Absolutely. I've seen studies where small, fast growing fish can consume 10+% of their body weight per day every day. For many tropical fish, that drops off rapidly and significantly as they mature past an 1" or 2". The studies I've seen of adult fish have never stratified them enough to discern a specific pattern at say 65%, 80%, or 90% of adult size. They simply indicate that "adults" need much less food for maintenance. Maintenance in the studies meant: able to sustain current weight. The figures varied from as low as .75% to 1.8% of "adult" size. There are wide ranges by species and of course temperature plays a big role.

A 6" oscar weighs around 63 grams while a 12" oscar weighs around 510 grams. So if the former is still eating 10% of its weight, it's still 40% less than the latter eating 2% of it's weight. Another factor is that food conversion ratios are typically better at younger ages so it takes even less food to grow when small.

The issue for the 12" oscar is this: to go from 12" to let's say 14", means going from 510 to 811 grams. If the 12" oscar uses 1.5% of his food for maintenance (no growth), he needs ~8 grams a day for maintenance. If he only is fed exactly 10 grams per day, it would seem like he'd only take ~(811-510)/2=150 days to get to 14", but he'll never reach that, since at 13.1", he will be larger and then only getting enough food to reach and maintain at 13.1". In practice, he'll take 4 months to reach 12.8" and another 4 months to get to 13.1" because every time he gets bigger the same 10 grams is being used more and more for maintenance and less for growth.

In theory then, anyone who sets and maintains a specific feeding regimen, can inhibit growth in an adult by feeding just enough until maintenance is reached. In the wild rapid and slow growth is somewhat normal for many species where food is plentiful, then becomes sparse. Many species will slow down growth, then when much more food is around they resume growth. But, if the food never becomes plentiful again, growth essentially stops.

One can't of course expect that an adult oscar will grow as fast as a juvenile. A 3" oscar that adds 1" has grown 33% in one month. A 12" oscar will never be able to come close to that, but a 12" oscar that is fed at or near a maintenance level will also never be able to grow much more as it simply lacks adequate nutrition to do so.


Consider wild oscars: the largest wild oscars weigh in at around 1700 grams, so they must be consuming at least 25 grams (dry weight equivalent per day.) But if their food is fish and insects (which only come in wet weights), then they must be consuming closer to ~70 grams of actual food each day. The oscars we have as adults have the ability to consume more food, because they can get it in dry form (sort of like dehydrated high nutrition food packs the astronauts get.)

Drstrangelove said:

Another factor is that food conversion ratios are typically better at younger ages so it takes even less food to grow when small.

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Oops meant to delete this. I know I've seen this reported somewhere, but I don't know what the results were and I can't find this in my links.

Very good posts Drstrange. I do find thru water monitoring that my big Oscars do create 4-5ppm nitrate per day. This is feeding every 2nd day. One factor that should be addressed by future and current Oscar owners is the nitrate ceiling before doing water change. Allowing nitrate to reach 40 on a regular w/c regimen is far too high. Assuming you have reached 40 and do a 50% change, you have only dropped to 20. I always recommend 20 as a ceiling, and I personally strive to keep nitrate at 10 or less as an average. I'm also of the opinion that as the fish ages, say beyond its half life span, metabolism slows greatly, thus requiring smaller amounts per feeding. Fish will still grow, but require less feed to do so. Concur?

I've never looked for or saw anything regarding fish metabolism and age. I agree however that a fully mature fish will need less food than a fish that is almost as large but still growing, so the effect is somewhat similar, but for different reasons.

As for the 40 ppm nitrate level, I just chose that as an arbitrary target. I agree I'd like my levels to be much lower, but nitrates don't affect all species equally. It's probable that some species benefit a lot more from very low levels (under 10 ppm) than others, so it's difficult to make a blanket statement that all fish need a low level. My opinion is if it can be achieved and there are no serious barriers (like time or money) one should strive for it, much like the fact that humans can ingest food with bacteria, but we should strive not to as much as is reasonably possible.

If low nitrates are the goal, pothos plants are a must have IMO.

4-5ppm nitrate per day out of one fish is massive! Perhaps I underestimated how messy they are. I never monitored water parameters that closely when I last kept Oscars, about fifteen years ago, but they thrived in my 150g along with heap of other fish on 40% weekly changes. Not sure what the long term effects may have been though as I replaced them with uaru after about a year.

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fin level every week, all tanks, when my oscar was in a 90 it was fin level twice a week to keep nitrates at or below 20ppm, at 24ppm it starts doing internal dmg over time. Oscar in the 180 now along with other fish, I see no reason not to do fin level every water change, if you can not afford the water bill to supply the best water quality you can, do not have fish. get a tarantula, feed once a month, live a long time, and only need to remove the sucked up dead cricket or mouse.

What's your biggest tank pops? I don't have the time, nor money (electricity), nor water reserves (house is supplied by rain water) to do "fin level" water changes. Should I get rid of my healthy thriving fish even though my nitrates only rarely hit 10ppm?

IMO, if you need to do fin level changes, its time to buy a bigger tank. If you have the resources and the inclination go for it, but it's certainly not necessary in all circumstances.

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