Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The smallest fish can be stocked more densely than larger fish, even when the difference in size is not as extreme as the mistaken example above. A fish that is twice as long as another fish is also twice as tall and twice as wide. It has 8 times the oxygen demand, and produces 8 times the waste, so needs more water to keep the oxygen available, and dilute the waste. A fish that only grows to 1" produces a lot less waste than a similar shaped fish that grows to 2".
The 1" per gallon guideline works pretty well when the fish are 2" and under. It was never meant to answer stocking questions for larger fish. Comparing 10 Neon Tetras to one Oscar is not using the 1" per gallon guideline correctly, and then saying the guideline does not work. If I start with a mistaken or misstated concept I can say a whole lot of ideas are false.
The 1" per gallon guide does work to suggest some limit of stocking that will still provide the oxygen the fish need, and enough water to dilute the wastes, as long as the fish are small enough.
It says nothing about the social issues of these fish. You could fit several male Bettas in a small tank, and they would have enough oxygen, and their waste would be diluted, but socially it would not work.
It says nothing about the activity level of the fish. You can put a Zebra Danio (about an inch long) in a gallon of water, but the activity level of this fish makes it not work. These are more examples of applying the guideline incorrectly, then saying the guideline is at fault.
If you are thinking about Ember Tetras and Licorice Gouramis you can physically fit a lot of them in a tank. They are very small fish, very small oxygen demand. Very small waste (ammonia and CO2) production.
If the Tetras and Gouramis you are thinking about are Pacus and Kissing Gouramis, then all bets are off. They would not even fit in the tank.
For a small tank you might think about something like this:
(1) Honey or Dwarf Gourami
(6-12) of some Tetra or Rasbora that only gets 1" or so long. For example up to 12 Embers or Dwarf Rasboras or other really small ones, or 6 Harlequins, Glow Lights, Neons or similar size schooling fish. A small group of Cherry Barbs (1M + 2-3F) could work.
(1-2) Otos or (6) shrimp for algae.
"How many fish fit" is not just about how many bodies you can stuff into a glass box, but how well you can maintain that environment when a problem hits. Under great conditions you can keep a lot of fish in a tank. But what happens when the power goes out? No water circulation, poor light (less photosynthesis), no heat...