15 Beginner Aquarium Mistakes You Should Avoid


When you are just starting out as a new aquarist trying to maintain the condition of your aquarium can be challenging.

There are so many things you need to keep an eye out for; this means there are a lot of mistakes that you can make without knowing.

Mostly these beginner mistakes are made because of a lack of experience.

In today’s article, we are going to look at the 15 most common aquarium mistakes and discuss how you can avoid making them.

1. Adding Fish Before the Aquarium Is Ready

Common mistake. We fall in love with little things before considering if we have space for them or not.

You buy your new aquarium and you’re excited to add fish and set the tank up right away. If you add fish before your water gadgets are stable, it’s very likely your fish will die.

Before you add your fish, your tank must have stabilized.

This means you need to complete a nitrogen cycle and water parameters (pH, temperature, hardness) must be stable depending on the type of fish you picked.

Only after these steps can fish be added to your tank.

2. Buying a ‘Small’ Aquarium

A great myth amongst amateurs is that a small aquarium is easier to maintain than a larger aquarium.

True to a certain extent.

I would generally recommend beginners find a tank between 30 and 60 gallons.

So why is it kid of wrong to start with a small/nano aquarium?

In a smaller tank, there is less water which means that the condition of the water can change very quick.

In a larger aquarium it is far easier to maintain the water parameters and quality.

3. Accidental Poisoning

Believe it or not it’s actually incredibly common for beginners to poison their fish by making the water toxic.

The main causes for this are:

  1. Failing to understand the nitrogen cycle
  2. Excessive feeding and waste left in tank
  3. Too many chemicals used unnecessarily

Perhaps the worst case of accidental poisoning I’ve come across is due to the filter being turned off at night.

Each time the filter is off the bacteria will start to die due to a lack of oxygen. This can will ultimately end up killing your younger fish.

4. Not Using Live Plants

This depends on your tank but in a freshwater environment I would consider live plans a necessity.

Most beginners see plants as complex and something which doesn’t benefit the aquarium. This couldn’t be further from the truth.

The biggest benefit of plants is that they help prevent algae, because they eat the same nutrients.

Live plants also help to oxygenate the water.

5. Using a Cheap Starter Filter

You may have noticed that many aquarium starter packs come with filters that are barely big enough; they usually turn the water over 2 or 3 times per hour. This isn’t enough and it can be dangerous for your pet’s health.

As a rule of thumb, you should find a filter that turns the water over at least 4 times per hour.

The good news is you can’t over-filter your water so it’s best to have a filter too big rather than too small.

6. Not Routinely Testing the Water

Unfortunately, aquariums aren’t “set it and forget it”. They require constant maintenance, especially new aquariums.

With new aquariums you should be testing the balance of the water daily, and with established aquariums monthly.

You should be checking the:

  • PH levels
  • ammonia
  • nitrites
  • nitrates
  • water hardness

Also I should note that if a fish unexpectedly dies, you should also check the water parameters.

7. Adding Too Many Fish

This is a similar mistake to adding fish to a new aquarium.

Adding too many fish to an aquarium at once can destabilize the water chemistry and can cause the nitrogen cycle to stall.

A good rule; only add up to 3 fish at a time to your tank.

Once you’ve added the fish to your tank, wait until you’ve completed a nitrogen cycle and then you can add more.

8.  Not Understanding the Nitrogen Cycle

One of the most confusing aspects of fish petting for a beginner is the nitrogen cycle.

Beginners tend to go one of two ways: they either completely avoid it and ignore it or stress too much over it. Whilst it is slightly complex, once you get it down, it is not difficult and it is just something you take in your stride.

All you need to know here is that failing to encourage and perpetuate the nitrogen cycle will result in your fish dying.

9. Not Having Enough Patience

Most of us lack Patience.

Whether it is waiting to add some fish to your tank, waiting for the nitrogen cycle to complete, or waiting for the growth of beneficial bacteria, chances are you will be waiting and waiting!

You will soon learn that having fish requires patience by the bucket load.

10.  Keeping Too Many Fish

Overstocking the tank is a very common mistake that beginners make.

It can cause problems with your filtration system and as a result can make maintaining your water parameters near impossible.

You may have been heard that you can stock 1” of mature fish per a gallon of water. That is totally wrong.

Instead you should use the 75% rule. This means that you multiply your tank capacity by 75% and this tells you how many fish you can keep.

Let me show you how this works with a 30 gallon aquarium. You multiply 30 by 75%; you end up with 22.5. This figure is the total length of the mature fish you can keep. So, in a 30 gallon tank you can keep up to 22.5” of mature fish.

11. Overfeeding

Without a doubt this is the biggest beginner mistake. It’s hard to know when fish are hungry, in-fact they always appear to be hungry and beg for food.

Generally, if you keep feeding your fish, they won’t stop eating. This is precisely the trap beginners fall into. They feed their fish because their fish keep eating.

This produces a lot of waste and can mess up the nitrogen cycle.

When you start, you should feed your fish once a day; this will prevent over feeding. Whatever food you fed to them they should be able to eat within five minutes.

12.  Insufficient Water Changes

As a general rule, you should look to do a 15% water every week.

Beginners often neglect this and think they can get away with less frequent larger water changes. Weekly water changes are crucial to keep the tank clean and healthy.

Poor water conditions cause fish stress and most likely increases their susceptibility to disease.

13.  Blindly Trusting Pet Stores

Let me preface this by saying there are some great fish stores out there. However some general pet stores have been known to provide erroneous advice.

Before you visit the store you should do at least some online research so you can establish if they know what they do or not.

Unfortunately just because your local pet store told you that keeping a Dolphin in a 30 gallon tank is ok, it doesn’t make it true.

Make sure to double-check and even triple-check the information they tell you, either here or in a book. It doesn’t matter where you check just make sure you do! A second or even a third opinion won’t hurt.

14. Lack of Biological Filtration

When you’re picking your first aquarium it’s a commonly held belief that an air-stone and a pump will provide enough filtration. Incorrect.

Regardless of the size of the aquarium you need a biological filter. Small aquariums can use the cheaper hang-off-back style.

You need a filter to ensure the growth of beneficial bacteria.

Also as a tip, remember when cleaning use aquarium water and not contaminated tap water. The chlorine in tap water will destroy your live bacteria- not the best.

15.  Mixing Incompatible Fish

Many beginner fish owners think that all pet fish get along and you can keep any species you want together. Wrong.

First, many fish need different conditions to live in. At a basic level this is freshwater versus saltwater, but also different water parameters.

Secondly, a lot of fish are carnivorous which means they will attack other fish.