Its time for "Fun with Math" or "Why 25% PWCs Aren't Enough".
Lets say that your aquarium nets 20ppm of nitrates a week. Your number will likely be different, but for the purpose of this example I'll use 20.
So, you have 20ppm nitrate and do a 25% PWC
20ppm - 25% = 15ppm nitrate
Assuming you have no nitrates in your tap water, you are now at 15ppm nitrate.
Now a week as gone by and the tank has generated another 20ppm nitrate.
15 + 20 = 35ppm nitrate
Time for the weekly 25% PWC
35ppm - 25% = 26ppm nitrate (Ive actually rounded DOWN here, so no complaining)
So 26ppm nitrate after the PWC
, but a week (and another 20ppm) later, your at 46ppm nitrate. Time for a 25% PWC
46ppm - 25% = 34ppm (again, I'm rounding down... its actually a little higher).
Another week goes by and now your at 54ppm nitrate.
Time for a 25% PWC
54ppm - 25% = 40ppm (rounded down)
One week later and its at 60ppm nitrate.
As you can see, by only removing 25% of your water/nitrates every week, your total nitrates are going to slowly increase over time. I only went out 4 weeks and the nitrates went from 20ppm to 60ppm... that's triple!
You might say "But I only net 10ppm nitrate every week." That's great. It means that the process will take longer, but it will eventually increase to a level that you probably don't want.
You might say "I do 50% PWCs every week". Also great. But it's also just delaying the inevitable.
Your best case scenario is that you are netting 0ppm nitrate every week. This means that your plants (or whatever) are absorbing 100% of the nitrates produced every week. This is pretty tough to do, but if you can manage it then your nitrate problem is solved. I'd still recommend weekly PWCs though, since you'll want to keep your GH
from slowly increasing from evaporation.
Your next best case scenario is that you do a 100% (or as close as you can get) PWC
every once in a while. This lets you get rid of much of the excess nitrates as possible and allows you to start the equation over with a much lower starting number.
I do weekly 50% PWCs, but on the 4th week of the month I do one that's around 90%. I leave just enough water to cover the fish.
Be careful about doing such a large PWC
when your nitrates have already reached a high number. Fish slowly get accustomed to the nitrates levels (this doesn't mean it's not unhealthy) and a sudden drop can shock the fish. Better to do several smaller daily PWCs until your nitrates are back down to a manageable level.