If one is new to fishkeeping, stocking is best looked at not by numbers but rather by water clarity. Slowly add some fish. Then see how it goes. Keep looking down the length of the tank and judging how clear the water is. If the water looks even a little bit cloudy do not add any more fish till the cloudiness clears up. Don’t worry about numbers. Gage the stocking by the health of the tank. How clear is the water? When the water is clear, add more fish. Cloudy water? Wait a while, till the cloudiness or “dullness” clears. Then you can add more fish. Simple!
Stocking and Opinions
The topic of stocking is largely OPINIONS, not experimentally derived scientific facts. Let me repeat that:
The Topic of Stocking is Largely OPINIONS
So do what YOU want to do with YOUR aquarium. If you want some guidelines which are largely just my opinions, read on.
The Science
Note that there is some science here. And the science says that VERY VERY HEAVY stocking of aquarium tropical fish is quite healthy and does little harm to tropical aquarium fish. The science is based on university research on the zebrafish and can be found in the following article:
13.2. Calculating Stocking Ratio
My Opinions and Recommendations
IN MY OPINION, the following stocking ratios, based on the volume of fish, are perfectly acceptable for a 100 gallon tank:
| Total length of fish | Light stocking | Recommended 100% level stocking | Heavy stocking * |
|---|---|---|---|
| 1in 2,5cm | 181 | 630 | 1890 |
| 2in 5cm | 28 | 93 | 280 |
| 3in 7,6cm | 10 | 32 | 97 |
| 4in 10cm | 5 | 17 | 50 |
| 5in 12,7cm | 3 | 11 | 33 |
| 6in 15,3cm | 3 | 9 | 27 |
| 8in 20,3cm | 1 | 4 | 11 |
| 10in 25,4cm | 1 | 2 | 6 |
| 12in 30cm | 0 | 1 | 3 |
| * ONLY with very heavy filtration | |||
For a 50 gallon tank the stocking numbers are:
| Total length of fish in centimeters | Light stocking | Recommended 100% level stocking | Heavy stocking * |
|---|---|---|---|
| 1in 2,5cm | 90 | 315 | 945 |
| 2in 5cm | 14 | 47 | 140 |
| 3in 7,6cm | 5 | 16 | 48 |
| 4in 10cm | 2 | 8 | 25 |
| 5in 12,7cm | 1 | 5 | 16 |
| 6in 15,3cm | 1 | 4 | 13 |
| 8in 20,3cm | 0 | 2 | 5 |
| 10in 25,4cm | 0 | 1 | 3 |
| 12in 30cm | 0 | 0 | 1 |
| * ONLY with very heavy filtration | |||
For a 25 gallon the stocking numbers are:
| Total length of fish in centimeters | Light stocking | Recommended 100% level stocking | Heavy stocking * |
|---|---|---|---|
| 1in 2,5cm | 45 | 159 | 492 |
| 2in 5cm | 6 | 23 | 90 |
| 3in 7,6cm | 2 | 8 | 24 |
| 4in 10cm | 1 | 4 | 12 |
| 5in 12,7cm | 0 | 3 | 8 |
| 6in 15,3cm | 0 | 2 | 7 |
| 8in 20,3cm | 0 | 1 | 3 |
| 10in 25,4cm | 0 | 0 | 1 |
| * ONLY with very heavy filtration | |||
For a ten gallon the numbers are:
| Total length of fish in centimeters | Light stocking | Recommended 100% level stocking | Heavy stocking * |
|---|---|---|---|
| 1in 2,5cm | 18 | 63 | 189 |
| 2in 5cm | 3 | 9 | 28 |
| 3in 7,6cm | 1 | 3 | 9 |
| 4in 10c | 0 | 1 | 5 |
| 5in 12,7cm | 0 | 1 | 3 |
| 6in 15,3cm | 0 | 1 | 2 |
| 8in 20,3cm | 0 | 0 | 1 |
| * ONLY with very heavy filtration | |||
For a five gallon the numbers are:
| Total length of fish | Light stocking | Recommended 100% level stocking | Heavy stocking * |
|---|---|---|---|
| 1in 2,5cm | 9 | 31 | 90 |
| 2in 5cm | 1 | 4 | 14 |
| 3in 7,6cm | 0 | 1 | 4 |
| 4in 10cm | 0 | 0 | 2 |
| * ONLY with very heavy filtration | |||
Look out with the heavy stocking columns for the ten and five gallon aquariums. The filters for small tanks are always very small cartridge or air operated sponge filters. These filters are just totally inadequate to support heavy stocking. So small aquariums should not be heavily stocked. If one puts something like a very small canister filter on a ten gallon, one can think about heavy stocking. But I’ve never seen that done.
To get a good number with mixed sizes just takes some common sense approximating. Start with the largest fish and work your way down in size. Say you have a 50 gallon (189 liter) and want to stock at the maximum recommended level.
Say you want 10 three inch fish. The 10 three inch (7,5 cm) fish is very roughly about half the quotas for “Maximum recommended”. That leaves 50% which could be 23 two inch (5 cm) fish (50% of 47). But lets say its ten three inch (7,5 cm) (roughly 50%) and 12 two inch (5 cm) (roughly 25% of 47). Then you would have room for one fourth (25%) of the 315 in one inch (2,5 cm) fish or about 80 one inch fish.
So it is 10 three inch (7,5 cm), 12 two inch (5 cm) and 80 one inch (2,5 cm) fish in a fifty gallon (189 liter).
That is all very rough. A more thorough analysis involves using the weight of the fish and gets all complicated totally unnecessarily. The easy way to do it is just to add small numbers of fish till the water turns “dull”. Then stop adding till the water becomes crystal clear. Then add more fish. Easy.
Now these numbers, especially the “heavy stocking” numbers, will absolutely send some “fish police” into absolute hysteria and get this site kicked off many Facebook forums. Remember, the heavy stocking levels are only “ MY OPINIONS” (not “rules”) based on WELL FILTERED AQUARIUMS WHICH HAVE LARGE FILTERS WITH GOOD MEDIA THAT HASN’T BEEN FREQUENTLY CLEANED!!!!!
There are ways to determine the amount of media and the amount of filtration needed for a given loading of fish. They can be found in the following link:
In the in depth articles below I give my OPINIONS on the topic and the OPINIONS of others. It is all a rambling set of OPINIONS so unless you want to get really bored, stop right here.
Stocking in Greater Depth
Hopefully the following links can put some common sense into this subject:
13.2. Calculating Stocking Ratio
And the common or narrow bodied goldfish should be stocked as a five inch fish.
This is delved into in this link:
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