As a guide, I change 300-400 litres per week out of 750 (max fill) litres split into 2-3 changes, this just about keeps things under control
(11 clowns and a Bristlenose)
(2x FX5 @4400litres per hour max total)
(Air stone in operation)
(PH 6.8, soft @26'C)
Bigger fish create more nitrogenous waste so more work is required by you to handle it.
Also larger biomass = more oxygen consumption, a similar thing with temperature! warmer water carries less dissolved oxygen (clown loach should be in warmer water and consequently there should be much more oxygen input. These fish do not tolerate poor conditions very well.
The total flow of your system should be 5-10 times turnover per hour, you can use additional power heads or extra canisters.
Hanging at the top is a dead give away for O2 deficit (99% of the time)
Clouded eye usually points towards nitrogenous waste levels too great.
You want 0 ammonia 0 nitrite and less than 50 mg/l nitrate, ideally 0 nitrate. Protracted periods outside of these levels will most certainly see your clowns out of shape.
Make sure you feed them blood worms as a regular dietary requirement, this prevents skinny disease.
The silver sharks will have a similar large bio load which should be taken into account, weakest fish die first, while clowns are tough they are not hardy.