# of fish in a tank

[reefer777]

I have a 75 gal. tank. What is the maximum number of fish I should have in there or does it really matter.

[monkeyprof]

Well it depends on the adult size of the fish, the amount of waste they put out, they aggressive nature ( or not), and if there will be corals. But you could safely put in 6-7 fish as long as they all are not huge fish or maybe 12 small fish.

For Example
2 clownfish
1 blue tang
1 lawnmower blenny
1 anthias
1 chromis
3 gobies

[artful-dodger]

It definitely does matter!

The question itself is somewhat scary...the goal should never be to see how many fish you can keep (in my opinion) but for the fish (and inverts) that I do keep, how close to perfect can I keep their environment?

Some other necessary background questions are: Is this a fish only tank? Is this a reef tank with primarily corals/inverts? What is the filtration system(s) configuration? What type/size fish are you considering?

If you are doing fish only, then the numbers may be quite a bit higher than if this is to be a reef tank. Large fish, of course, are bigger eaters and therefore bigger polluters than small fish. (Triggers and moray eels, for example, tend to be pretty messy and tear their food up and only consume part...the rest is waste.) Fish (as a very general rule) tolerate water quality issues much more readily than corals. Fish waste (and uneaten food) is processed by the filtration of the tank through a cycle of ammonia, nitrites and finally nitrates. Having a larger number of fish will push more waste through this cycle...the end product (nitrates) don't cause much problem for fish, but can easily kill more sensitive inverts.

There have been quite a few "rules of thumb" over the years about how many inches of fish can be supported. Don't believe any of them. Three inches of a trigger will be probably 10 times the bioload of 3 1" clowns or neon gobies.

Also, you must make you plans based on the eventual size of the fish, not their current size. Stocking with 6 1" fish may seem like a good idea until all six are 4" long in a couple of years.

Okay, if you are talking about setting up a reef tank, you need to keep the nitrate levels low. There are two things you need to do to accoplish that. First you need a filtration system that can process/concume nitrates...that generally means a refugium with macro algae whtat will use nitrates as food and (along with light) convert them into plant matter that can then be harvested. Second, you need to keep the inputs to the nitrogen cycle as low as possible--that translates into holding down on the amount of food going into the tank and that, in turn, translates into reducing the number of fish that need to be fed.

So, if you are doing a marine fish tank, you will be able to stock more heavily. (as an example, I have a tank 1/2 the size of your 75. It it are a 12" snowflake eel, a 3" mandarin clown, a 3" dwarf angel and a 2.5" sailfin tang along with some snails and a large bristle star. So, in answer to your question, a 75 could comfortably support 8 good sized fish. (Incidently, the sailfin will be moved to my bigger tank when he gets a little bigger"...a 37 is way too small for even a medium sized sailfin.) But, I'm not supporting any delicate corals (only mushrooms and a few zoas and leathers) in that tank.

If that same tank were set up as stony coral reef, I would have four times the lighting and probably no more than 3 very small fish. The attraction of the reef tank is the corals...the fish are more of a decorative accessory.


G'luck (but don't try to push the limits...the recoil can be murder!)