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How many fish?
So often asked but not easily answered.
Decades ago it was shown how bio-load was, in essence, the criteria of fish capacity -- The bio-load carrying ability of the aquarium/system and the bio-load each fish puts on the system. But what average hobbyist could do the complex computations to get a reasonably reliable estimate? Could the average aquarist estimate body mass and would that be good enough? The inches-per-gallon scheme was used in by freshwater aquarists and it came over into the marine hobby. It was simple and really wrong at the same time!
Obviously marine fish length was not a means to determine what bio-load the fish puts/places on a system. Does the 4 inch long and thin Butterflyfish (e.g., Copperband) put the same bio-load on the aquarium as a 4 inch long and thick Harlequin Tusk? I think everyone sees the uselessness of using inches per gallon to determine how many fish-inches can be put into a marine aquarium. But there's nothing to really replace the inches-per-gallon scheme other than experienced aquarist's opinions and what appears to be subjectivity.
I knew that the mass, consumption (of food), wastes produced, etc. could be scientifically calculated. But this kind of calculation was not something the hobby could use. After all, the reason inches/gallon was being used was because it was simple. The most-used scheme would be the easy scheme.
We know too that it isn't the gallonage of an aquarium which determines the aquarium bio-load ability (capacity). The system configuration determines bio-load capacity. That is, the bio-filtration method, circulation, other filtration, substrate, skimmer, live rock, refugium, etc. So there had to be a similarly easy means of estimating the bio-load ability of the system.
Both the bio-load ability of the system and the assignment of bio-load to fishes (mostly) and nitrogen waste producing marine life, was something I worked out. I use that information.It's just not as simple as I wish it was, because there are so many different system configurations and types of fishes.
Then there is a somewhat intangible part to the scheme. This is the space a fish needs (either actual/real or from the fish's perspective). Some fishes swim a lot in the open ocean, others stay and guard specific areas. A mated pair of fish will want and need certain space that doesn't really relate to their true bio-load. Same with fishes like the Tangs. The sub-adults of these fish join large schools that travel for up to 25 miles per day. In captivity, extra space and distance in the aquarium is needed. So in the scheme I devised, such fishes are given 'normal' bio-load numbers, but are in a category of fishes that require a minimum of X type/size aquariums. Thus the bio-load unit scheme comes with a set of rules. Not so simple!
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