Over stocked for that filtration. General rule of thumb is shoot for 10x the turn over rate an hour. If those are rated at 90gph a piece, and you have 2, you're filtering that tank 1 time an hour at most... Not enough my friend. If you're new to sumps (I am -- just set up my first sump over the weekend) i would say go cannister. You cant go wrong. Shoot for 1200gph total in the end. You cannot over filter your tank.
As far as your LFS saying 50% is too much thats absolute shenanigans. I do 60-70% with no issues. The reason being is that the good bacteria does NOT live in the water, it lives in the filters and substrate. By draining the tank out you are not removing nor hurting the good bacteria. If the LFS tells you otherwise they are uninformed and greatly mistaken. That being said these are the same type of guys who sell RTC to the
With large water changes, Its more about altering and changing the water chemistry to dramaticly for the fish, at one time, not so much about removing good bacteria.
It all depends on how often you do those 50 60 70% water changes. If you change your water regularly then you will probably can get away 50% or so.
If you dont do water changes on a fairly regular basis, and then all of a sudden change 50 or 60% of your water at one time, you can kill fish, depending on the fish that you keep.
Some can tolerate it better than others.
You are changing the water chemistry for the fish to much at one time, thats why your LFS said 50% is to much at one time. I agree with them.
I never change more then 1/3 of my water at any given time. I learned the hard and expensive way.
