10 Things You Shouldn’t Put In Your Fish Tank

If you’re at the stage where you’re ready to set up your fish tank, you’ve probably done a lot of research.

You most likely know what size tank you’ll need, how to set it up, how many fish and which fish species you’re going to keep in your tank.

If you’ve bought your fish from a reputable supplier, they will have given you plenty of guidance on how to care for your fish and what to include in the tank.

What most people don’t tell their customers though is what not to include in the tank.

This is just as important, if not more important than knowing what things to include in your tank.

If you unknowingly put certain things into your aquarium, you could very well put them in danger and some items can be fatal for fish.

But what are these things?

1. Plastic

Many fish shops sell plastic toys, like Nemo or SpongeBob, to include in your tank.

When plastics are left in water for long periods of time they can release potentially toxic chemicals into the water, so they should be avoided in fish tanks at all costs.

There are a number of exceptions to this, if the plastic has been graded food safe. This is often displayed with a triangle made up of three arrows.

However, avoid one time use plastics such as water bottles.

Recent research undertaken on plastic bottles left in cars, shows that chemicals can leach if used for a prolonged period of time.

Often the plastic toys you can purchase from fish stores are painted, and the paints may not have been sealed. Ensure when you buy plastic toys that you find out whether or not they have been sealed and are safe for your aquarium.

Unsealed, painted plastic toys will release toxins into the water which could poison your fish.

Even with sealed plastic, there is always the chance that they could chip or split and then release toxins in the water.

We advise that you make the fish tank as similar as possible to natural habitats and avoid plastic when possible.

2. Ceramics

Similarly to plastics, ceramics can also release heavy metals and chemicals into your aquarium water as the protective glaze dissolves.

Some ceramics are ok to use in your tank – if they are marked as ‘dinnerware safe’ they will be fine for your tank.

As a basic rule of thumb with ceramics, if it’s not safe to eat off, don’t put it in your aquarium.

There is also evidence to suggest that lead is present in some pottery which is a health hazard for both humans and fish alike.

Highly decorated items and glazed terra cotta clay are likely to contain lead, there are kits available to test whether lead is present but unless it’s vital you include it, it’s best to leave it out of your tank.

3. Anything Consumable

Aside from the food you provide your fish, no other consumable objects should be placed into your tank.

Fish chew and bite a lot. If you place rock in there which is loose, it’s possible they could chew off a small piece which could be fatal. Or, toxins could be released into the water.

Make sure you know all the side effects off adding things to your tank, there are even some plants which are harmful for fish so do your research thoroughly.

4. Wood

Driftwood in Tank
Don’t put un-treated wood in your aquarium.

Many of the wooden ornaments that you find in pet shops have been treated, so they are fine to include.

But you can’t just put un-treated wood into your aquarium. This can alter the chemistry of the water in your tank.

Driftwood is fine to add and is a nice addition to an aquarium.

If you’re going to include driftwood you should choose a hard wood. This is because hard woods take a long time to decompose and won’t have a drastic impact on the water.

5. Beach Sand

Whilst treated sand can make a great substrate, beach sand is generally polluted and will contain chemical residue which can harm your fish.

Logic may tell you its fine to include some in your tank; it’s the fish’s natural habitat.

However the ocean is a vast collection of water which is constantly renewing – your fish tank is not so big so just a few chemicals can drastically harm your fish.

You can put sand in your fish tanks; just make sure you buy it from a reputable place.

If you’re determined to use beach sand in your tank, it can be done but you need plenty of patience and time.

It involves lots of soaking the sand, and changing the water, rinsing and letting the sand settle in the tank water. This process is lengthy and can take 2-3 weeks.

6. Shells, Corals and Rocks

Shells in Aquarium
Don’t put shells in freshwater aquariums.

We should note here that we’re only talking about freshwater aquariums for this category. Shells and corals add calcium to the tank and this is not needed in freshwater tanks.

Basically anything that will cause chemical changes in the water should not be put in.

Seashells, amongst other items, can change the PH hardness which will cause difficulties for your fish and it’ll be harder to maintain the tank.

7. Anything Degradable

If you put something into your tank which can degrade, it will alter the water which is far from ideal.

Depending on the object, again, this can realise toxins and chemicals in to the water, or it can make the water become dirty.

While we’re on this topic – it’s important to remember that fish food should only be left in the tank for 5 minutes and the remainder should be removed to reduce the amount of waste left in the tank.

8. Your Hands

It is fine to put your hands into your fish tank as long as they are not contaminated. By this, we mean, hand soaps and creams should be thoroughly washed off.

Soaps and other residues left on your hands can be harmful to your fish.

Before putting your hands into the tank, wash them with soap and rinse them off a couple of times to ensure all the soap is off.

9. Anything Sharp

Don’t put anything sharp into your tank. This includes glass with sharp edges, or painted glass, objects or decorations with sharp edges.

Paint can flake off objects and poison your fish.

Remember, this doesn’t just apply to items you have found, make sure you also check your shop bought items frequently too.

One cut to your fish could lead to infection if not quickly treated so best just to avoid anything sharp altogether.

10. Large Fish Breeds

This is perhaps an obvious observation to those who are more experienced, but it is one that must be covered.

Even if you have a 100 gallon tank, there are still many breeds of fish which are not suitable for your tank.

An example is the Iridescent Shark, a large catfish which is a very active swimmer and can grow up to 4 foot long in the wild.

Always do your research into fish breeds and their requirements before you add them to your tank.

Not only is buying the wrong fish a waste of money, but placing fish in a tank that is too small for them, will inevitably lead to early death.


To conclude, if you have an item you want to include in your fish tank, ask yourself these questions:

  • Will it release toxins into the tank?
  • Is it degradable?
  • Is it sharp?
  • Is it consumable?
  • Is it going to cause chemical changes in the water?

If you’ve answered yes to any of these questions, do not put the item in your tank.

Let us know in the comments section below what items you have placed in your aquarium…

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