Basically I've recently bought a tank, 250l which started off as 90% fresh water, (pre heated, aerated, etc). But over the past couple of weeks I've found it very difficult to keep clear. I did a 40-45% water change yesterday and already it looks very murky and green, and I can't figure out why the water is getting dirty so fast. I have a Fluval Powerhead 402 in there as well as a Fluval 203 canister filter. Any advice on how to clear this up besides continuous water changes? The last one clearly didn't work..