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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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This may not be too helpful, but if it were me I would take the 1 inch per gallon "rule" as a guideline more than a steadfast rule. It has its merits but is certainly not the end all be all. For instance, your 10 inch Oscar probably wouldn't do so hot in a ten gallon tank, but it meets the "rule." I would say the same thing could go for wide bodied fish who are wider than they are long. Take an Angelfish at a completely made up 5 inches, you could stick 8 in there according to the "rule" but I think many would argue that's over stocked.

Whether or not your filtration could handle a given load is a different question altogether and since I'm not familiar with it I'll skip it entirely. But I will say it likely depends on your water change schedule. You could probably get away with running an under powered filter or over stocking a tank if you were willing to do more frequent water changes. Again, probably not something that's recommended.

People frequently mention AqAdvisor to help determine appropriately stocking levels. Again, it's not the be all end all but it is a good start. Between common sense, that site, and searching for others who have done what you want to do, you'll get pretty darn close to a solid stocking situation.
 

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Yes the size of the fish makes a difference. Big fish certainly put up more bioload in the tank than groups of small ones. For example I believe my school of 15 black ruby barbs, about 25 inches of fish, produce less poop than when I had one 9 inch goldfish in the tank. Most of that goldish was tail at that.
 

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1" per gallon guide works only for fish about 1-2".

Here is why:
A fish that is a similar shape, but twice as long as a smaller fish is also twice as tall and twice as wide.
This means 2 x 2 x 2 = 8 times as much ammonia and CO2 wastes, and 02 requirement.

A fish that is a similar shape, but three times as long as a smaller fish is also three times as tall and three times as wide.
This means 3 x 3 x 3 = 27 times as much ammonia and CO2 wastes, and 02 requirement. A rough guide just does not stretch this far!

The 1" per gallon guide is only valid for waste production and oxygen need. It says nothing about the social needs, activity level or anything else about how best to keep the fish.

Research the fish you are interested in.
 
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