If you’ve been around the aquarium hobby for any length of time, you’ve likely heard this advice. “Do not overstock your fish tank”. But what exactly does that mean?
When I hear that an aquarium is overstocked, my first thought is “compared to what”? Compared to the ocean? Well yes, even a 2 inch fish in a 200 gallon tank is overstocked compared to the billions of gallons of water that wash around your average coral reef. But does that mean your fish tank is actually too crowded? Maybe, maybe not. Personally I have two definitions of overstocking…
Do you have too much bio load for the filtration to keep up?
This one is pretty objective. If your bio filter cannot keep up with your fish and animal load, then your tank is overstocked. This likely means you have ammonia and nitrites, and almost certainly dead fish.
While this is a huge problem, fortunately there are ways to get around this, such as feeding less, adding additional porous filter media for beneficial bacteria to grow on, using a denitrifying external filter, or simply adding booster bacterial material on a regular basis.
Do your fish have enough space to live happy lives?
This one is a lot more subjective. Essentially, do your fish have enough space to swim, eat, and not fight too often? I have a 7 inch Hippo tang (Dory, for those of you who have watched Finding Nemo) in my 90 gallon reef tank. I also have a 5 inch Purple tang (that’s his picture in this post) but I’ve had these fish for 10 long years, and they are the only two fish in the tank (along with various anemones, corals, and invertebrates). So is my tank overstocked?
If you read literature on Hippo tangs the answer is most definitely “yes”. The literature says that Hippo tangs love to swim, which is true, and that they grow up to a foot in length (in the ocean yes, but I’ve never seen a Hippo tang bigger than 8 inches in captivity). So technically my tank is overstocked, but in reality both Tangs have room to swim, hide, and eat. They’re fat and (seemingly) happy, have never had any disease, and are well into their second decade of life on this planet. So you be the judge. Is my reef aquarium really overstocked? I would say “no”.
For aquarists wondering if their tank is overstocked, I would say don’t bother worrying too much about it. In general, most small fish (clownfish, blennies, gobies, damsels, etc) should be fine in a tank of at least 2 feet in length. Tangs need a tank at least 4 feet in length and large angelfish need a tank at least 6 feet in length. Go slowly with your fish stocking and ALWAYS quarantine new arrivals for at least 3 weeks before adding to your main display tank.
What’s far more important than overstocking is good water quality and a good diet.