ok so I have a 120 gallon with three baby oscars and one baby pacu and a baby plecos, and one small electric blue johanni(or something like that),and one betta(soon to be fish food I'm sure). First off I know that the pacu will outgrow this tank and am planning on upgrading later on. Now I have two emperor 400 power filters and plan on gettin an undergravel filter soon to help circulation. The tank is a new setup I keep the temperature 75-80 degrees. my question is when I vacuum the gravel every week how much water do I need to change. and would it be better to vacuum syphon style or with a canister filter so that the clean water goes back in the tank and then do water change? and how often should I replace the filter cartridges in my filters?