Doing some math.

I was thinking a few days ago, how partial water changes would end up polluting a tank in a very long term.

Only considering nitrates, please, read as follows:

"My fishload raise my nitrate reading 10ppm evey week. At the tank's first water change time, it reads NITRATE 10ppm. I do a 50% water change with clean water (0ppm) and now my tank nitrate concentration is reduced to one half (5ppm.)

Next week, tha tank should be 15ppm. I left 5ppm remianing last week and the fish added their share. Now, that tank has NITRATE 15ppm. My 50% water change lowers it at 7.5ppm.

Next, it's at 17.5ppm, and the water change lowers it to a half, 8.75%

After a few weeks, nitrate will reach 20ppm, and your water change lowers it to 10ppm. Then, fish will raise it to 20, and so on."

My fisrt impression was pollution would keep raising, but after a little help from my excel spreadsheet, to my surprise, it stabilizes.

Now I know that regular water changes are good. I know this post is now irrelevant, but decided to share it. I think it's a good prove how regular water changes are good for your tank.

I found a pattern and no matter what conditions you have, the tank will stabilize on time as long as everything (tap water nitrate, fish nitrate production, plants nitrate consuption, water change schedule, etc...) remains the same.

Example, in my first post I stated an unplanted tank, with a nitrate production of 10ppm per week, and a 50% water change schedule with tap water that would contain no nitrate. According to my calculation, the tank would become stable in 11 weeks.

If we add plants that are able to "eat" 5 ppm of nitrate per week, then it would be stable also at week 10.

If we raise the nitrate production at 30ppm per week: stable at week 13.

However, this will change dramatically if the tap water contains nitrate or if the water changes are reduced. Going back to the first post, the same tank with a production of 10ppm of nitrate weekly, but with a 25% water change will take about 30 weeks to estabilize. If the tap water has a 5 ppm of nitrate, it will take also 30 weeks.

I know all these calculation are hypothetical (¿sp?). I calculated it using 1/100 ppm, which is not really useful in the fishkeeping hobby, since nitrate concentrations are normally measured in 10ppm increments.

Also, I forgot to state that the larger your water changes are, the tank will stabilize at a lower "normal" nitrate concentration.

When you do 50% partials, you nitrate reading will almost be always the sum of the fish production + nitrate in tap water.

If you use to do 25% partials, it will be higher. If you plan doing 75%, it will be lower than that.

Well... I spent almost 2 hour gettin that conclusion.

I think you got the best avatar-personality combination of the entire internet.