dapellegrini
Aquarium Advice FINatic
As I have started to flesh it out, I think the old rule is still a good base. Surface area would be more relevant for a standard setup, but as soon as you start playing with variables, every rule breaks down.
I think people need to understand two things (1) tank capacity - how much life can the system sustain (2) and max system load - how much life will you be attempting to sustain assuming your fish all survive to adulthood.
Chasing the first number, take you tank size, say 75-gallons. Adjust out gravel, so we have say 70-gallons. Now, find your filters GPH rating. Adjust it down for a more accurate number. Say it is rated at 350gph, we will call it 300gph. That is about 4.3 turns per hour. Assuming 3 turns per hour is standard, the rest gives you a bit more room to support life, so 1.3 is about 33% increase in efficiency from our base-line. So we can adjust from 70 to ~93.
Now, do you have a planted tank? I mean live plants of course. If you have a medium or dense level of plants that frequently pearl (meaning they give off so much O2 that the water column reaches saturation), then give yourself another 25% (of the original number), which brings our current total to 110. (I am also thinking that you might get an adjustment here for an airstone?)
Now how much water is replaced each week? If you only replace water monthly, divide it out to get a weekly number. 0-20% water change a week is probably a wash, so I wouldn't count any economies there. If you do 25-75% a week, I would count that in, such that 50% PWC a week would give me another 50% (of the original 70), brining my total to 145.
Here is the break down:
Tank size: 75 gallon
Adjust for gravel: 70 gallons
% efficiency of filtration over 3 turns per hour: 33% (of 70) = ~23
Plant density and happiness: 25% (of 70) = 17.5
Weekly water change (over 20%) = 50% (of 70) or 35
TOTAL tank capacity number: 145
Then when counting your fish, make a list. Don't measure their current size. Instead find a good internet site and mark down their adult size. Then decide if each fish is a small (skinny, like a neon tetra), medium (like a Cory cat) or large (like an oscar) body type. Use a 1, 2 or 3 multiplier, respectively.
So 5 neons = 1.5-inch adult size x 5 x 1 (small body type) = 7.5
3 cory cats = 3-inch adult size x 3 x 2 (medium body type) = 18
Etc.
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So let's take a typical, not planted, low-tech setup and see how the numbers come out.
Tank size: 20-gallon
Adjusted for gravel: 18-gallons
GPH Filtration over 3: 0% (standard HOB filter at 60GPH adjusted down from 100GPH because you don't clean the thing out often or well)
Planted Bonus: None (plastic plants and a sunken ship)
Weekly water change: 25% every 4 weeks = ~7% a week so we get a 0% here
Total tank capacity index: 18
Then look at what you can put in there:
4 neons = 4 x 1.5 x 1 = 6
2 corys = 2 x 3 x 2 = 12
Perhaps if I gave you some extra leeway for that bubbling treasure chest, you would have enough for one more small fish.
Thoughts? (If you actually read the above)...
I think people need to understand two things (1) tank capacity - how much life can the system sustain (2) and max system load - how much life will you be attempting to sustain assuming your fish all survive to adulthood.
Chasing the first number, take you tank size, say 75-gallons. Adjust out gravel, so we have say 70-gallons. Now, find your filters GPH rating. Adjust it down for a more accurate number. Say it is rated at 350gph, we will call it 300gph. That is about 4.3 turns per hour. Assuming 3 turns per hour is standard, the rest gives you a bit more room to support life, so 1.3 is about 33% increase in efficiency from our base-line. So we can adjust from 70 to ~93.
Now, do you have a planted tank? I mean live plants of course. If you have a medium or dense level of plants that frequently pearl (meaning they give off so much O2 that the water column reaches saturation), then give yourself another 25% (of the original number), which brings our current total to 110. (I am also thinking that you might get an adjustment here for an airstone?)
Now how much water is replaced each week? If you only replace water monthly, divide it out to get a weekly number. 0-20% water change a week is probably a wash, so I wouldn't count any economies there. If you do 25-75% a week, I would count that in, such that 50% PWC a week would give me another 50% (of the original 70), brining my total to 145.
Here is the break down:
Tank size: 75 gallon
Adjust for gravel: 70 gallons
% efficiency of filtration over 3 turns per hour: 33% (of 70) = ~23
Plant density and happiness: 25% (of 70) = 17.5
Weekly water change (over 20%) = 50% (of 70) or 35
TOTAL tank capacity number: 145
Then when counting your fish, make a list. Don't measure their current size. Instead find a good internet site and mark down their adult size. Then decide if each fish is a small (skinny, like a neon tetra), medium (like a Cory cat) or large (like an oscar) body type. Use a 1, 2 or 3 multiplier, respectively.
So 5 neons = 1.5-inch adult size x 5 x 1 (small body type) = 7.5
3 cory cats = 3-inch adult size x 3 x 2 (medium body type) = 18
Etc.
*********************************
So let's take a typical, not planted, low-tech setup and see how the numbers come out.
Tank size: 20-gallon
Adjusted for gravel: 18-gallons
GPH Filtration over 3: 0% (standard HOB filter at 60GPH adjusted down from 100GPH because you don't clean the thing out often or well)
Planted Bonus: None (plastic plants and a sunken ship)
Weekly water change: 25% every 4 weeks = ~7% a week so we get a 0% here
Total tank capacity index: 18
Then look at what you can put in there:
4 neons = 4 x 1.5 x 1 = 6
2 corys = 2 x 3 x 2 = 12
Perhaps if I gave you some extra leeway for that bubbling treasure chest, you would have enough for one more small fish.
Thoughts? (If you actually read the above)...
. 
