Continuous vs Large water changes

[simonh]

Hi again Randy,

I am just designing my new system an I am thinking of placing a large container in my garage to hold about 25% of the system water volume and putting in some small pipes so I can do water changes without having to bring barrels into the main living room. I got thinking on which would be the best method of performing the change.

Is it best to have say a small pump continuously running in effect changing a small volume of water in the system each day (in total say 20% per month). Or is it best to once a month just drain out 20% of the water from the tank and refill ?

I must admit that my head went a little fuzzy when I tried to do some calculations... but I think I worked out that one off change would dilute/raise any ions more than a continuous change which would tend to drift. Any thoughts? Or maybe even a combination of continuous with twice yearly large changes?

Many thanks,
Simon

[Randy Holmes-Farley]

Simon:

There are two competing effects:

1. Larger changes of the same amount of water are always more efficient. One 80% is better than 2 x 40% is better than 80 1% changes.

2. Animals might react negatively to large changes. Or at least, you need to spend more effort thinking about matching temperatures, etc.)

The differences when you are doing small changes are fairly small. Continuous changes vs 2 10% changes per month is a minimal difference. I'd go with whatever is easiest to engineer.

It's fairly easy to model mathematically. Since you are a software type, excel can be used to do a good job.

Put the number 100 in a cell.

Multiply it by the water change (say 0.8 for a 20% change).

Do that as many times as you have changes.

So 10 5% changes would be:

100 x 0.95 x 0.95 x 0.95......

(or just 100 x 0.95^(10))

The number remaining is the percent of something that remains after the changes.

So 10 changes of 5 % would be about 60% remaining.

One 50% change, of course, has 50% remining.

So the larger change is a little better, but it isn't a huge difference.

For a continuous change there is an equation that I don't have handy, but 1% changes give about the same answer.

[simonh]

Thanks for that Randy. I should go wear my dunces hat and stand in the corner considering I write software for Microsoft Excel :D