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Hi! right now i have an acrylic (truvu) 40 gallon tank set up that is moderate to heavily planted, and has just successfully achieved a strong cycle after 3 weeks of fiddling with the chemistry. it has a fluval c 3 HOB filter, and i am in the process of deciding what community fish to add. i mentioned the filter because ive read online that the amount of fish you can keep correlates to the strength and efficiency of the filter. of course i constantly see the "1 inch per gallon" golden rule quite literally everywhere, but i have become somewhat confused due to conflicting advice i learn.
does the golden rule change when pertaining to smaller or larger fish? (like does overstocking with tetra have the same consequences as overstocking with something bigger like silver dollars or angelfish)
to elaborate, is breaking the rule with larger fish more dangerous than with smaller ones?
also, does my current filter allow for me to tweak the principle? (due to the fact that my research has led me to believe the 4 stage filtration in the C filters is quite good)
thanks!
 

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The 1" per gallon guide is good only for fish to about 2" long, though something really skinny like a Kuhlie Loach could be OK.

The 1" per gallon guide is just a guide for waste removal and oxygen supply: A small fish (1") produces a certain amount of ammonia and CO2, and requires a certain amount of oxygen.
A fish that is a similar shape, but twice as long is also twice as wide and twice as tall. That is a volume of 2 x 2 x 2 of the smaller fish. That is, 8 times the waste, 8 times the oxygen requirement.
As long as the fish are no larger than 2" it is OK. You could probably overstock on 1" fish and still be OK.

The 1" per gallon guide says nothing about:

Social issues- schooling fish need to be kept in a large enough group. Don't keep 1-2 Ember Tetras in a 1 gallon tank even though they only get 1" long. Similarly, do not keep solitary fish in a group, unless the tank is really big enough for them each to have their own territory. Fish that are social in certain numbers (pair bonding, harem breeders and so on) should be kept in groups of the correct numbers of males and females, and in a tank of the correct size.
Shy fish should not be kept with more aggressive or active fish. Research your fish!

Food related- a) A predatory fish that specializes in eating other fish can almost always handle a fish that is up to half its own length. So, do not keep a predatory fish with smaller fish.
b) Some fish are highly specialized and need a special diet. Certain Cichlids, for example need a high vegetable diet. Do not keep them with fish that need a high protein diet.

How the equipment is part of this:
Keeping the water moving is the key to proper gas exchange. No matter if you use a filter, power head, bubbler or other equipment to circulate the water, it is important that the equipment not stop. In case of a power outage a lightly stocked tank can hold out a lot longer than one that is overstocked.
A tank with more surface area can hold out a lot longer than a tall tank, because gas exchange only happens at the water surface. I have found I can stock a lot more fish in my 4' long, shallow tanks (45 gallons, 4 square feet or more of surface) than I can in my 45 gallon tank that is 1' deep x 2' tall x 3' long (3 square feet of surface).
The amount of surfaces in the media in the filter, is important. All the filter media: sponges, floss, not just the bio media hold nitrifying organisms. These organisms need high oxygen levels. In a planted tank there will not be as big a population of nitrifying organisms because the plants use a lot of the ammonia that is produced by the fish. Still, that population of nitrifying organisms is part of the system, and must not be compromised.

So...
If you have more fish in the tank then be prepared to take care of the fish and microorganisms in case of a power outage.
If power outages are common in your area you may want to be cautious about over stocking any tank.
 
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