Are there diminishing returns on water changes?

[JoeBanks]

I was thinking of setting up an automated water changing system that would replace 1 gallon of water per hour continuously on my 180 gallon tank. This would replace roughly 100% of the water once a week.

I was wondering if doing something like this would give my a noticable difference in plant health over doing two 25% water changes per week. Is there a point where more water changes won't make a difference?

 

[quenton]

Re: Are there diminishing returns on water changes?

If nothing else -- you will complicate your nutrient dosing schedule.

Lets assume you or someone can figure out how much to dose for each gallon change (since the water you are putting in has no nutrients), then you need some way of auto-dosing. Even if you dose daily its going to be a nuisance, and daily you are going to have 24g replaced so your nutrients will be done 20% or something.

If you can figure it all -- I guess it would work -- not sure what the advantages might be -- I guess its automated -- but maybe you could automate a weekly or bi-weekly change??

 

[JoeBanks]

Re: Are there diminishing returns on water changes?

I would have an automated doser if I go with this setup. I would fine tune the dosing to maintain constant levels of each nutrient.

The ultimate purpose of going with this setup is to eventually have a planted discus tanks with very stable and clean water conditions for the benefit of the fish.

 

[Tom Barr]

Re: Are there diminishing returns on water changes?

They use a similar thing for algae nutrient controlled expeiments, they are called chemostats. They are PITA to keep going, even though the concept is simple.

As new water is added, it's enriched.

You do NOT get 100% changes, you change a portion of the each 1 gallon fraction of the 180 gal tank volume per hour or continuously(You can do a nice(or not so nice depending on your views towards math) calculus equation to describe this relationship vs an infinite series(since the series would approach zero).

I can replace 50% of my water 2 x a week, but there is still build up(although not as much). The time frame for build up is relatively short.

You may want to run the dosing based on a week dosing routine.
Say adding 30ppm NO3 a week etc.

I'd have a dosing system add everything once a day in a pulse.
That would not dilute the ppms down for some time and supply the plants fairly well for a few hours daily.

If you have a reservior, that would be best, make up a 50-100 gal batch and add ferts to the make up water.

Regards,
Tom Barr

 

[vidiots]

Re: Are there diminishing returns on water changes?

I made an excel spreadsheet layed out like a calandar, for figuring out the max build up of nutrients asuming that none is used or added to by the living things in the tank. It also asumes a consistant water change schedule and consistant amount changed each time. If all of my math is correct I think I noticed a pattern for making a simple formula to come up with the answer. I also assume that the dosing is done after the water change and not before.

I noticed that with the 50% weekly water change the max buildup was 2x the weekly dose, or 14 times the daily dose.

A 50% weekly water change works out be equal to a 7.14% daily water change.

I did the math asuming a 5% daily water change, and the max build up was 20 times the daily dose.

I did the math asuming a 10% daily water change and the max build up was 10 times the daily dose.

If this is correct your 1gal per hour change on a 180gal tank should be 24 gals per day or a 13.333% daily water change and should result in a max build up of 7.5 times your daily dose.

The formula I come up with from this is:
MaxBuildUp = 1 / (DailyWaterChange% / 100)
MaxLevel = MaxBuildUp x DailyDose

Another thing I noticed is that it takes a few weeks to reach the max buildup, and the number of weeks varies with the percentage of water changed. Taking less time for a larger percentage of water changed, and more time for a smaller amount changed. Also as you can see the max build up is larger for a smaller amount of water changed and smaller for a larger amount of water changed, which makes sense right?

This formula works only if you use the same unit of time for both the water changes and dosing. For math purposes if you use weekly water changes, figure weekly dosing, if you use daily water changes use daily dosing.

Anyone wanna check or correct my math?

 

[Tom Barr]

Re: Are there diminishing returns on water changes?

It depends on how you dose, if you dose continuously via the make up water then no............if you dose all at once during the day, then yes, the model seems about right.

I would suggest a daily once a day dosing.

So if you dosed 3ppm NO3, the max build up of 7.5 x 3 = 22.5, not bad.

But that assumes there is no uptake, you'll likely sit in lower end of things.
Add 25%, 50% and 75% and 100% uptake and see how your model changes.

You'll likely have to adjust the dosing routine to get better growth since you will be at the lower end. An ocassional pulse of higher nutrients will likely help.

I would not do this higher pulse all the time, just once a week perhaps.

Regards,
Tom Barr

 

[bogongmoth]

Re: Are there diminishing returns on water changes?

According to this page:
http://www.angelfish.net/DripSystemcalc.php

24 gallons a day dripped into the tank will change 12.48 % of the Tank's water in one day, 23.41 % in two days and 60.68 % of the Tank's water in a week.

 

[Tom Barr]

Re: Are there diminishing returns on water changes?

Ahhh a cool calculator.

That's a perfect thing for seeing what it'll do.

Regards,
Tom Barr