Choosing fish and stocking your tank

It’s easy to think that choosing fish for an aquarium just means browsing your LFS for fish whose appearance you like. In reality nothing could be further from the truth and stocking appropriately is both a science and an art.

Choosing fish – first dispel the myths!

To begin we need to disregard the myths that pervade the hobby therefore I’ve listed just some of the more pervasive myths below:

Do fish only grow to the size of their tank?

The idea that fish self-regulate growth and stay a convenient and healthy size for their environment is still the number one myth despite having been disproved long ago. There is some truth in it, n that if you put a young fish of a large species in a small tank, it may not grow to a normal adult size. However, this is not a beneficial self-regulation, but a disordered growth pattern caused by the lack of space and usually associated poor water quality and diet. This is called stunting. Stunted fish are not healthy and may suffer numerous deformities, damage to their immune system and compromised functioning leading to an early death.

Are water hardness and pH irrelevant if I choose fish from a local shop?

This is probably the next most commonly encountered myth. Some will tell you that neither hardness nor pH matter if you buy locally. They believe one of two things: that becasue the fish are alive they must have adapted to your local water, or that the fish have been bred in local water. The truth is that almost all of the fish for sale in your local shop will have been bought from wholesalers who source wild caught or farmed fish bred in water similar to their natural conditions from abroad. They cannot adapt that fast considering the millions of years it took for them to evolve to suit the conditions they are naturally found in. You can read more on this topic in my article on aquarium water hardness and pH.

Do I really need to cycle the tank first? Can I cycle with hardy fish?

Yes, contrary to what you may have been told, it’s essential to fishless cycle the tank before buying fish. If this isn’t done, your fish will be slowly poisoned by their own waste. In the past, tanks were cycled over a few weeks by adding one or two so called “hardy” fish every week until fully stocked. Ammonia from the fish eventually started a cycle and as the fish didn’t die it was thought to be OK to do that. We know now that surviving a cycle is a very unpleasant experience for the fish who will potentially suffer ammonia burns to the skin and gills, oxygen deprivation due to nitrite affecting their blood and preventing it from delivering oxygen to their cells, long term damage to liver and kidneys, increased susceptibility to infection by diseases and parasites due to immune system damage and a shorter than normal life span. Fishless cycling substitutes ordinary household ammonia for the fish and will ensure the ere are already sufficient beneficial bacteria present to deal with the fishes’ waste before they ever go into the tank. You can read more about this on these pages:

  • “The Nitrogen Cycle“
  • “What is Fishless Cycling“
  • “Fishless cycle – how to cycle an aquarium with ammonia
OK, but can I skip the fishless cycle and add fish after 3 days as long as I use bottled bacteria?

The answer to that is a definite “No, don’t waste your money and fishes’ lives doing that”. Bottled bacteria haven’t been proven to work effectively in fully establishing a cycle. The bacteria contained in most of these products are not the same as the ones we need to grow as those cannot survive in a bottle with no food or oxygen. The products were developed to protect those “hardy” fish in the early stages of cycling “fish in”, but this seems to have been lost along the way and the idea that a tank will be instantly cycled and ready for a full stock list of fish in just 3 days has taken hold. However, all they really do is process small amounts of ammonia and nitrite depriving the bacteria we really want to grow of their food source. In addition, if added to a tank with no fish, the bottled bacteria will have no food source (ammonia) and will die. Even with the right bacteria, it takes several weeks for an initial small colony to enlarge sufficiently to handle all the waste from a fully stocked tank.

Now back to the subject at hand….

Choosing fish

Hopefully, you’re doing or have done a fishless cycle, you know the pH and hardness of your tap water and you’re ready to start choosing fish for your tank.

Questions about this? Click here.

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