[demonsquirt]

You say you have 20 times exchange but ill bet you count the skimmer and or canister.
Do you use any powerheads?
And just doing a small water change wont do much.
You need to get the proper water test kits with nitrate and phosphate and DKh readings.
Do your tests around the same time and in the morning is best.
Get the levels and then if you have high unwanted levels then a 35 % water change followed by weekly 25% changes untill wanted levels reached. Then 10% weekly or 25% monthly would be ok.