The beauty of minimalism
Let me try to sow a new seed — the majority of tanks look better with a reduced number of fish. Considered stocking choices and a couple of outgoing, bright shoals will vastly outshine a ‘pick and mix’ tank that’s overcrowded. Just ask the aquascapers…
While this rule suffices for sedentary species, it hits the wall when it comes to active and migratory fishes. Of course, a fish that spends its whole life in a state of inactivity can be happily kept this way (think Banjo catfish, or small marine frogfish that stay motionless), but any of the South American catfishes that travel many miles each week (or day) will be doomed to repetitive circuits of their tank, swimming in an endless loop.
While it’s not the easiest thing to research, activity levels need to be factored in when considering if a tank is overstocked.
The long formula you’ll never (want to) use
If you are adamant that you want to stock a tank to its capacity, there is a long-winded calculation that can help you work out if you have enough filtration to cope. It works best if your tank or pond is at 25°C, with a pH of 7.2 and alkalinity of 100mg/l, as variation to any of these parameters will affect the outcome. You’ll need to weigh the amount of food going in to the tank, which in turn means you’ll need to weigh all of the fish (the easiest way to do this is to weigh a bucket of water, then add your fish to it, weigh it again and subtract the first number from the second one). And you need to weigh the fish as you’re aiming at feeding 1% of their bodyweight a day for maintenance.
Then you need to know the protein content of the food that you’re using. Flake foods typically run at around 35-45% protein.
Because we know that roughly 50% of the protein from food comes out as wasted nitrogen, we need to factor in a rate of 0.16g nitrogen per gram of protein, and then multiply the lot by 1.2 to calculate the amount of total ammonia produced.
Following so far? Okay, So assuming I feed 3g of flake to my fish each day, and it has a content of 40% protein, my formula would look like this:
3g food x 40% protein x 50% wasted protein x 0.16 nitrogen per gram protein x 1.2 ammonia per gram of nitrogen. That would give me 0.115g of ammonia being produced in total by the fish in my tank each day. (The easy way to do this calculation is 3 x 0.4 x 0.5 x 0.16 x 1.2 = 0.1152)
Next, we need to discover just how much biomedia we need to convert that ammonia, and that’s where things get really grey. Biomedia comes in many forms with different surface areas, but we know that in ideal conditions 1m3 of plastic bioballs with a surface area of 200m2/m3 can convert 90g of total ammonia a day.
The catch comes in working out the surface area of your own media (for example, Kaldnes K1 media has 500m2/m3 of surface area, a fluidised sandfilter has between 5000-10000m2/m3 of surface area, while specialist aquarium media like Siporax boasts around 270m2 surface area per litre of media.) If you’re able to source the surface area of your own filter media of choice, you then have a chance of working out how much media is required to convert all of your daily ammonia. Then, from a filtration perspective at least, you’ll not be overstocked.
I never said it was easy, but if you really want to know if your filter is coping with your stocking density, and you’re prepared to crunch the numbers, there’s a way to find out. Oh, and did I mention common sense? If you get a positive test for ammonia or nitrite despite having a fully cycled filter, that’s a good sign of overstocking right there.