That’s abt 600 gph which is good. Assuming you packed your baskets w media for both aerobic and anaerobic bb, things should be ok. I’m a fan of using multiple filtration methods.....sump, canister, and hobs.
What I’m going to put forth is meant w all due respect to Liz, as she does indeed know her stuff. Other members might diss me, for frequent massive WCs do seem to be the “norm” for many at this great site, and that’s fine. My thoughts are just another perspective from another Discus hobbyist. If I had to do 30% WCs daily, I dare say I’d have to leave the keeping of Discus.
While WCs are really good for water perimeters, ultra filtration can ease the frequency some, especially with adult fish...imho.
You might try adding a sump and or hobs to increase your water cleansing. If you can maintain low nitrate levels w the other factors in balance, then the need for daily 30% WCs will not be necessary. 9 fish in a 90g tank is a fine ratio of 10g per fish or so (can’t forget the Cories) assuming you don’t overfeed. My 75g tank has a FX4 canister, two hobs (400 and 200), two anaerobic canisters, and a 15 gallon sump w sock, bio balls, and a 3 foot long refugium with water sprite. I have a heavy bioload of 10 Discus in it w five Cories and my NO3 levels are between 0-5. I do weekly 40-50% WCs and all is good. I realize that 10 is too many (7.5g per fish), but compensate with all the filtration and two light feedings a day. My tanks are very thinly planted with 1/2-3/4 inches of coarse sand. My 55g tank is setup the same, except a Fluval 406 canister instead of the FX4, but with 6 Discus in it.
Again.....many Discus people do the daily WC thing w great success and I’m merely showing what works for me. Lastly, having only one type of filtration is a bit risky (especially w power outage issues and canister only filtration). When you have multiple filtration methods, the others can pick up the slack some should the canisters fail or when cleaning out the canisters.
Gators111’s concern for pathogens is very real. Doing a QT on new arrivals is critical and do try the “Hero” fish routine. I’d hate to see your well established tank be jeopardized by adding new fish hastily.
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This pic only shows 2/3 of the tank.
Respectfully,