Here's a easy to understand "common sense" guide on stocking levels:
Hope this helps - Ed
Goldfish produce waste and ammonia based on their weight, not their length. Their weight dramatically increases as they grow in length. So, the larger they get, the more waste is created. Goldfish produce approximately .3 of their weight in waste per day!!!
A shocking example of waste produced by a goldfish:
If one was lucky enough to have a 10" goldfish, it would have an approximate body mass of 9.70 ounces.
A single 2" fish only weights .08 ounces, so even if you stocked ten 2" goldfish, they would only have a combined weight of .8 ounces. The 10" goldfish will produces .91 ounces of waste per day, but the ten 2" goldfish combined produce only .24 ounces !!!
Size 1" Approx. Weight .01 Waste .0003
Size 2" Approx. Weight .08 Waste .0024
Size 3" Approx. Weight .26 Waste .0078
Size 4" Approx. Weight .62 Waste .0186
Size 5" Approx. Weight 1.21 Waste .0363
Size 6" Approx. Weight 2.09 Waste .0627
Size 7" Approx. Weight 3.33 Waste .0999
Size 8" Approx. Weight 4.96 Waste .1488
Size 9" Approx. Weight 7.07 Waste .2121
Size 10" Approx. Weight 9.70 Waste .2910
Size 12" Approx. Weight 16.76 Waste .5028
So what does this mean in layman's terms?
If you have searched the internet for goldfish care, I am sure you have found many sites "preaching" the "10 gallon rule". Although the 10 gallon rule is a good place to start, it is by no means accurate for long term success, nor should it be the definitive answer for all goldfish keepers.
As one can see from the example above, the bio load of one single 5" goldfish is equal to fifteen 2" goldfish!!!
For even short term success, one should have at least a 10 gallon tank, but do not beat yourself up if you have something smaller.
We all need to use common sense when it comes to stocking our tanks. If you have a 5" plus goldfish, you should not even think about keeping it in anything smaller than say a 20 gallon tank, primarily so the fish doesn't feel cramped. On the flip side, if you have a 2" rescue fish, you should not feel bad if the only tank you have is a 5 gallon.
For maximum goldfish health and clean water, what one really needs to consider is the bio-load their filter can handle.