Why a weekly 50% WC is better than two weekly 25% WCs

[ScatMan]

I don't think anybody pointed out the fact that under scenario number one in the very first post, it starts with minimum nitrate = 30ppm.

The section that reads:

• Minimum nitrate = 30ppm
• Maximum nitrate = 40ppm
• Average nitrate = 35ppm

SHOULD READ

• Minimum nitrate = 20ppm
• Maximum nitrate = 30ppm
• Average nitrate = 25ppm

SO your average nitrate IS actually lower with biweekly water changes. Big fail on that post, unless I'm missing something here....

you're missing that op started wc schedule at the minimum concentrations allowed by each wc. in scenario 1; even if you start at 20ppm, concentrations will gradually increase to 30ppm min and 40ppm max. ppm's will never get back down below 30ppm on that schedule with 25% bi-weekly wc.

 

[MonsterMinis]

All I know is when nitrates start creeping up over 20ppm on my knifes tank... I don't wait for it to hit 40 before I do my 25% water change.. when he's rubbing all over obviousely in discomfort. over-all the nitrates may be slightly higher in general but there are never excessive amounts the fish is exposed too. and if I never let them get over that middle difference.. doesn't matter really how the water changes are done only that my NItrates stay below a toxic level. which imo is the real answer they should be looking at, not how to save on the water bill.

Useing sensitive fish as example. I like the smoke filled room reference... either way it's not healthy to expose them to excessive amounts... so to reiterate...

"You can never do to many water changes" is the correct answer.

 

[calioutlaw1a]

Thanks Scatman for helping me out here. I didn't realize this would be so difficult.

Would it be better to increase the mineral content and decrease the hormone concentration twice a week, or once a week? Use your own body as an example. Would it be better to drink a glass of water and go to the bathroom twice a day, or would it be better to drink two glasses of water and go to the bathroom only once a day?

In these scenarios there is the same amount of water removed (50%) over the same amount of time (1 week).
So to answer your question: I would rather relieve 50% of the volume in my bladder and have it fill back up over the course of the week so I only need to pee for the last half of the week, rather than relieving 25% of the volume in my bladder twice in the week which would feel full throughout the whole week.

Like I said, you're missing the big picture. Instead of a fish living with "x" amount of nitrate every 3.5 days with 25% water change twice a week, it will lives with "2x" amount of nitrate every 7 days with 50% weekly water change. It's not the fluctuating nitrate is harmful to fish. It's living with nitrate for longer period is harmful to fish.

No. With a 50%wc, the nitrate is reduced to 20ppm whereas with each 25%wc the nitrates are only reduced to 30ppm. The fish are exposed to LESS nitrates with one 50%wc. The numbers don't lie.

I don't think anybody pointed out the fact that under scenario number one in the very first post, it starts with minimum nitrate = 30ppm.

The section that reads:

• Minimum nitrate = 30ppm
• Maximum nitrate = 40ppm
• Average nitrate = 35ppm

SHOULD READ

• Minimum nitrate = 20ppm
• Maximum nitrate = 30ppm
• Average nitrate = 25ppm

SO your average nitrate IS actually lower with biweekly water changes. Big fail on that post, unless I'm missing something here....

You are
It states once equilibrium is reached, meaning once you reach a state where nitrate levels are no longer fluctuating from week to week. In scenario 1 equilibrium is reached where after each 25%wc nitrates drop to 30ppm, and increase to 40ppm after 3.5 days. Nitrates initially start at 20ppm and increase over several weeks to reach this equilibrium.

 

[dxdx]

OK, now I get it. I'm still going for the constant drip system on my 300, no way I'm doing manual wc...

 

[calioutlaw1a]

OK, now I get it. I'm still going for the constant drip system on my 300, no way I'm doing manual wc...

I envy you.

 

[jcardona1]

Here's a different spin on it, using one of the effective water change calculators.

http://www.hamzasreef.com/Contents/Calculators/EffectiveWaterChange.php

Assumptions: 120g tank

50% weekly water changes (60g)
After one month, you'd have 7.5g of the original water left, or 93.75% new water

25% biweekly water changes (30g every 3.5 days)
After one month, you'd have 12g of the original water left, or 89.99% new water

In summary, I doubt the 5g/4% difference after 1 month will make a big difference in your nitrate levels. Just do whatever fits in better with your schedule and your lifestyle. That's all there is to it really. We're just chasing ghosts here arguing about what's better.

 

[ScatMan]

Thanks Scatman for helping me out here. I didn't realize this would be so difficult.

no problem.

it seemed pretty intuitive to me, i didn't realize that this was such a poorly understood concept until i read the comments challenging your math.

apparently people needed to see this to understand it, so good job to you for posting!

 

[vfc]

vfc's charts are flawed because he changes the water every 3 days, NOT 3.5 days (aka 50% bi-weekly). the 3 day chart is more than 50% a week hence the appearance of a better option.



25% every 3 days does NOT = 50% per week

I can only use whole days on my spreadsheet due to the way I detect the WC day (I use the MOD function and when there is no remainder, that is flagged as the WC day).

Here is a 50% every 6 days chart vs. 25% every 3 days. The swings are greater with a 6 day WC. The average is higher for 3 day changes.

 

[ScatMan]

I can only use whole days on my spreadsheet due to the way I detect the WC day (I use the MOD function and when there is no remainder, that is flagged as the WC day).

Here is a 50% every 6 days chart vs. 25% every 3 days. The average is the same, but the swings are greater with a 6 day WC.

you have 5ppm in the tap for the 50% wc and 0ppm in the tap with the 25%.

fix that and it will be the same max ppm for both after they equalize.

*** never mind, looks like you caught it.

 

[Poloboy]

i do 2 times a week a wc of 25% to keep my water values the same