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Controlling the Population of Your Fish Tank


Once you have your aquarium all set up, with the right number of fish, the right plants and décor, you can just sit back and enjoy all the benefits of being an aquarium keeper – but then what happens when your fish start breeding?

If you have a perfectly balanced aquarium, an increase in fish numbers could cause major problems so what are the best ways to control your fish tank population, and what should you do if that population suddenly starts to explode?

Fortunately there are plenty of measures which you can take to either prevent over-breeding in the first place, or to handle excess fish or other creature populations if you start to see problems occurring in your tank.

But why is overpopulation such a problem in the first place? Surely the more fish the better; after all they live in huge shoals in the sea sometimes? Knowing how many fish you should have in your tank is an important part of fishkeeping. Let’s take a quick look at the damage which an over-populated tank can experience:

Why are extra fish a problem?

Although your fish breeding might seem like a sign they are happy and comfortable, unfortunately if they keep breeding they won’t stay that way for long. Some fish species breed very easily and they often have a high number of babies to increase the chances of survival.

That means you could end up with many more mouths to feed, a lot more waste being produced into the water and a very crowded tank, very quickly. The increased waste will put pressure on your filters and the extra organic materials in the tank will encourage algae growth.

So having extra fish can destroy your tank’s equilibrium very quickly, which is why it’s important to have some kind of population control measures in place from day one. Here are some ways you can help to reduce the risk of over-population among your fish:

1. Only buy one gender

When you buy your fish, stick to one gender, that way your risk of babies is very minimal. The best choice is to only buy male fish because if you buy all female, there is always a chance that one or two of them might already be pregnant when you buy them, so you could still end up with babies in your fish tank.

However, if you opt for a male only fish tank then you won’t have any problems with over populating. You will need to consider whether any of the breeds are territorial however as you don’t want to end up with aggressive males killing each other, either.

2. Buy egg-laying fish only

Look for egg-laying fish as they usually require very specific conditions in which to lay and hatch fertile eggs so you can make it difficult for them to be successful. If they do lay eggs they are not likely to survive and if you spot them you can always remove them from the tank before they get a chance to develop.

Live bearing fish tend to breed much more easily so are harder to control in this type of manner. Some popular egg-laying breeds include tetras and barbs. The ones to avoid, which bear live young easily, are swordtails, mollies and guppies.

3. Make the tank uncomfortable for breeding

It can be a delicate balance but as some fish are incredibly sensitive to their environment, you could try adjusting the temperature or ph levels just slightly, so they are uncomfortable and therefore unwilling to breed. This is a very tricky and delicate way of doing it and it might create health problems with your fish so it’s not the best option when it comes to controlling the fish population in your tank.

What do I do if I find baby fish in my tank? 

These are some of the ways you can avoid unwanted babies and over population from actually taking place, but nature will often find a way so what should you do if lots of unwanted fish do suddenly appear in your tank one morning?

The easiest option is to let nature do the work for you. With a fish population it is likely that many of the babies will be eaten fairly quickly by the other fish in any case. You can try removing hiding places, and adding a couple of aggressive fish into the tank to provide a natural control measure.

If your babies do survive and grow then you can always sell your unwanted fish to other aquarists, or anyone who would like some fish such as schools trying to set up a new aquarium – there are always people around who will be willing to give them a home.

What you should never do is release them into nature – non-native fish released into ponds or rivers and streams can cause havoc with the native animals and habitat and can be subject to fines and proceedings in some areas as well.

If you take suitable measures with your tank environment and the type of fish you purchase, you should be able to avoid over-population issues fairly easily, and if you do end up with excess fish why not just set up a second aquarium in another part of the house?

What about the other creatures in my tank? 

It’s not just the fish who can cause problems with over-population in an aquarium however. If you choose to have other aquatic creatures like shrimp or snails, these too can become prolific breeders and cause population issues.

With shrimp they tend to breed according to how much food is available so the population can be kept in control by limiting the food supply – also certain types of fish will eat up any baby shrimp on site, which can also help to keep the population reduced.

Snails can be the same, by keeping the food supply limited; shrimps and snails can compete for the food left over by the fish which will discourage them from over breeding and causing problems within the tank.

You can always remove excess snails from your tank and sell them or place them in a second tank – again, it’s not too hard to find new homes for aquatic creatures – talk to your local aquarist shop or stockist to see if they can help you rehome them.

Take measures to keep your aquarium environment balanced

As you can see, having a tank which has excess fish or other aquatic life can create havoc with your carefully created underwater world. Excess livestock will cause waste which can pollute the water, and will encourage the growth of algae.

Your perfectly aligned tank could very quickly go from a balance of fish, clear water and space, to one of too many fish, cloudy polluted water and an invasion of algae, unless you keep the population under control.

You could also find your tank over run with shrimp and snails if you do not take measures to keep their populations under control as well, making the tank very uncomfortable for your fish and causing many of the same problems in terms of excess waste and pollution.

The good news is if you follow the guidance here on helping to prevent breeding, you should be able to maintain a healthy, balanced aquarium fairly easily. Sticking to an all-male fish tank is a guarantee of no baby fish so that’s probably the simplest option.

What if I get male and female fish by accident? 

However, it’s always possible that mistakes will be made, particularly if you buy your fish from a pet shop rather than a specialist aquarist centre, as the staff there might not have the expertise to guarantee you only buy male fish.

Or if you prefer to have a combination of fish then opting for a couple of aggressive species which will eat the babies and control the population naturally, could well be the easiest option to take, ensuring nature takes its course and your tank is once again in balance.

If all else fails then you can be creative and set up another tank for your baby fish and creatures to be transferred into and perhaps have that one in a different room in the house, or gift it one of your friends or family members who have always wanted an aquarium but perhaps didn’t want to go through setting it up for themselves.

Alternatively, local schools or places like care homes or healthcare settings might be grateful for a donation of fish for their tanks, for education or relaxation purposes so if you wake up one morning and find lots of baby fish – don’t despair as there are plenty of options to resolve the issue.

As you can see, an over-populated fish tank will ultimately cause problems for your fish and result in a very unpleasant environment which will be bad for their health and could ultimately kill your fish so it’s really important to maintain a good balance within your aquarium, for the sake of your fish and to make your own life easier when it comes to maintenance and care.


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