How many fish can I have in a 75g+20g sump

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T-J

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The old school general rule of thumb is 1 inch of fish per 10 gallons. Having said that, it really is going to depend on the types of fish and what kind of aquascape and tank you have.
With the list you provided, you should be fine with those.

For reference, I have a 120g with a 40g sump, and I currently have 8 fish. A yellow tang, 2 clowns, 2 lightning chromis, flasher wrasse and 2 cardinals. I may add 1 or two smaller fish in the near future, but that's it.
 

ying yang

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Factors that i would be considering would be :
How much oxygen can get in system,surface agitation/protein skimmers etc etc.salt water holds less oxygen than fresh water and colder waters holds more oxygen also.
Size of fish ( i.e if juvenile or adult size,always best to go on adult size even if buy juveniles).
Swimming behaviours of fish,some like to stay perched on rocks,some hide in caves,some like to swim around constantly so need to cater the aquascape for every fish you have in tank with free swimming space or lots caves.
Aquascape also can let new algae growth form so algae eating animals are kept busy so less time on their hands to show agression to others.
But then territorial fish seeing other fish eating their food then could lead to aggression so fine balancing act.
Surface areas for the bacteria to live to make water safe/ none toxic.( bigger fish eat and poop more )
Tight fitting hood/ screened net or open top could influence your fish choices ( known jumpers etc) even though any scared/ spooked fish can dart into rocks down below or swim upwards into open water and jump right out the tank.
Compatability with fish - other fish, some are schooling fish and some are shoaling fish and some just do better being the only one of their kind in tank.
Flow in the tank ,some like or can handle faster flow in tanks and some will not.fish will always find area of tank that suits them best.
With the many factors to consider i dont think there is a one size fits all,as just a rough guestimate but if in doubt we can always do our research on each fish that we want ( size it grows/ tank size needed/ compatibility etc) and stock tank slowly leaving few weeks between each introduction and visually see how the fishes are acting in their little environment.
 

Shannon_Hill

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Many factors in determining this, but others have done a good job of pointing this out. I have a 75g as well that’s currently stocked with 17 fish. I do use a lot of nutrient controls (GFO & lanthanum chloride for PO4 and large refugium and NoPox for NO3).
 
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