Topping off freshwater aquarium with tap water?

[TwoHedWlf]

The only thing lost to evaporation is water, so don't add anything other than water; no added salts or other mumbo-jumbo that the tank "requires". Certainly, dechlorinate the top-up water.

Honestly, I can't recall the last time I have ever "topped-up" a tank. My tanks all have lids, and if evaporation has dropped my water level far enough to be noticeable, then it's already past-due for a water change.

Started to respond then realized this is a necropost from 2019. But, yeah, if there's enough evaporation to care, it's time for a water change.

 

[FINWIN]

Well I was thinking about the threshold or breakpoint in chlorination to where the residual would be negatively impactful to the fish.

The volume op has is topping off 5 gallons of water into 70 gallons making the total volume 75 gallons. I just threw a easy dilution factor of 10 because i can do it without any thinking but lets use OP's gallon.

5 gallons of 75 gallons is 6.67%
5 gallons having 4ppm
70 gallons having 0ppm
changing to liters will make it easier as a ppm is just a way to say mg/liters

topping off
5 gallons = 18.92705 liters still 6.6% of the tanks volume
initial volume having 0mg/l of chloramines
70 gallons = 264.979 liters
total volume
75 gallons = 283.906 liters

dilution formula is c1v1=c2v2

were left with
4mg/l * 18.92705 = c2 * 283.906
when solved is
.266 mg/l = c2 (final concentration)

So we know the tanks ppm or mg/l will be .266ppm for that 283.906 liters or for that 75 gallon tank. I couldnt find any scientific studies on chloramine levels and fish to show what levels are toxic that were credible but im going out on a limb and saying .266ppm of chloramines isnt that harmful. Were talking about .266 parts of chlorine per million gallons of water lol... The epa sets the maximum to be .4ppm of total chlorine which is a mixture of free chlorine and also combined chlorines, most municipal water supplies dont use straight chloramines. By the time it goes throughout the distribution center and reaches your house going through 100 year old pipes in the streets its not .4ppm as that is a significant chlorine demand. Only time the water supply uses .4ppm of straight chloramines is if its snow melt or head waters (first use), they will use uv to disinfect and then have .4ppm of chloramines leaving the water treatment plant. But again by the time it gets to your house its probably closer to 2-3ppm which drastically lowers the .266ppm in OP's tank. And even further lowers if hes in a place where they use free chlorine and combined chlorine.


Ive topped off like this this many times in the past years ago and those same fish are still with me +5 years nearing 10 years. Sure people probably lost sensitive fish to topping off a tank without any dechlorinator but its really isnt that dangerous. As i mentioned any activated carbon you have in your filters will render the chloramines inactive. Of course just dose the water your adding or whole tank and then add water is always recommended. But its not as dangerous as your leading on, they do act with organics, just not as readily as free chlorine does usually with nitrogen creating the organic chloramines. The debate isnt whether it reacts with organics (which it does creating organic chloramines) its if the small amount of chloramines that were diluted will harm fish. At that level most fish are fine and wont skip a beat.

*Reaches for Abacus* ?

 

[Arthur11]

There are some simple steps to be able to use tap water in the aquarium that you should know. To deal with harmful substances in tap water, you must first check some factors, such as pH, calcium carbonate, heavy metals, etc., in tap water and remove them. After removing chlorine and other unwanted metals from tap water, there is a good chance that there are still other harmful substances found in the water. Applying a balm is a necessity to get rid of this unwanted toxic substance.

The presence of chlorine in the tap water will surely hurt your fish. There are two ways to de-chlorinate the water. One is to let it sit for 24 hours to allow the salt to evaporate. The other way is to treat them with chemicals, specifically conditioner, to evaporate chlorine and chloramine.