40g inline prop tank
1 Yellow Tang - 4.5 yrs old (4")
1 Purple Tang - 4.5 yrs old (4")
1 Copperband Butterfly - 3 months (4")
2 Ocellaris Clowns - 4 yrs. old (2" each)
1 Hippo Tang - 4 months (3")
2 Firefish - 5 months (2" each)
Total = 23" of fish. I guess I could get a few more fish, eh?
The rule of 5"/fish per gallon of water is a very general/loose guideline. A lot depends on the fish's eating & swimming habits, territoriality, etc. 4" of Firefish is a lot "less" fish than 4" of Trigger.
, more "small" fish take up less "fish space" than the same amount of inches in larger fish. Larger fish - Tangs, for instance, need a lot of swimming/grazing room. Triggers, Lions, etc., have voracious appetites, and add considerably to a bioload. Blennies, clowns, gobies, damsels, etc., OTOH
, do not require large tanks, extensive swimming space, and are not very heavy eaters. .
Overall, you need to make sure the fish are not in a stressful enviroment (don't put a Tang in anything less than 4' long) - are not cramped/crowded, & have compatible tank mates. If you can do this, and also make sure that your filtration can keep up with your bioload, then you can push that 1"/5g rule.