If you have only just introduced your fish to your aquarium, they will likely go into hiding for a couple of days. This is totally natural behaviour. They need time to adjust to their new environment, check out their tank mates & get used to your routines.

It is also normal for some species of fish to change colour, or have their colours fade while they are stressed. If your fish does this when it is first introduced to the tank, please try not to worry. It is a natural behaviour.

Tank Mates

One reason your fish may be hiding is because you are trying to house it with inappropriate tank mates, just because a fish is listed as reef safe or a community fish, it doesn’t mean they will automatically get along with the fish you already have in your aquarium.

An example of this is Surgeonfish (or Tang’s as they are more commonly known). Though they are generally considered reef safe, they can be very aggressive towards each other, especially if they are from the same species of Tang, or they have a similar silhouette to each other. It’s normally best to pick Tangs that have a totally different body shape to each other.

Be sure to research all new members of your aquatic paradise before adding them to your aquarium, this will help ensure any future purchases are compatible with existing tank mates.

Putting a predatory fish in your tank will likely cause most of your fish to hide, not just the little ones, but anything that thinks it might be eaten by the predator. If you have just added a predator to your aquarium full of community fish, I suggest returning the predator or you risk losing all of it’s tank mates one by one!

Compatibility Chart
Compatibility Chart

It’s Natural

Some fish are naturally more tentative and used to hiding than others, for example the Torpedo Gobies I have in my aquarium only come out during feeding or if they think no one is looking at the tank. The second they spot you, they both dart into hiding. This behaviour is natural to them, but it is likely amplified because they are not in a large shoal of similar species fish.

Another example is the Royal Gramma, a beautiful fish, but it hugs the rockwork constantly and is never too far away from its “safe spot”. Midas Blennies are popular because they like to sit in a hole and poke their head from it, really, this is the fish hiding and being in its comfort zone.

Some fish are just naturally more wary than others, research into each purchase should give you an overall idea on the expected behaviour of your fish.

Midas Blenny in hole
Midas Blenny in hole

Sickness & Water Quality

If your fish is normally active & no new tank mates have been added then it’s time to consider that the fish is unwell or your water parameters are off somewhere.

I would always recommend tracking your water quality, this will allow you to keep an eye on any sudden dips or spikes in your vital parameters. Regular testing should include pH, alkalinity, calcium, magnesium and Nitrates. If these are all in check, it would be worth checking you Ammonia and Nitrite levels. These can spike if one of your inhabitants perishes.

Marine Velvet
Marine Velvet

If you can see the fish, try to monitor any changes in behaviour or condition. This could include the fish swimming erratically, gasping for air, being unable to balance underwater, cloudy eyes and many other things.

If you can spot the behavioural or conditional change, you can then research that condition to find out the best way to treat that ailment.

Squabbling

Within your aquarium there is a natural pecking order, sometimes this will need to be re-established. This will result in some squabbling between your fish. The more dominant of the fish will chase and bully the less dominant.

Most of the time, this will calm down, but there are occasions where one fish will bully another to death. Not allowing it to eat, quite literally beating it or keeping it so stressed that it just gives up and dies.

If you notice you have a bully, you do have a couple of options.

Clown Tang
Clown Tang
  1. Remove the bully and send it back to the fish store.
  2. Remove the bully temporarily and allow the bullied fish to establish a territory that it wants to defend. Hopefully once the bully is returned to the main display it has to work it’s way up the pecking order again and isn’t top dog straight away.
  3. Put an even bigger bully in the tank that will distract the first bully. (Not a trick I would recommend).
  4. Move your aquascape around to remove the bullies territory. They then have to re-establish themselves as the boss again from the start.
  5. Place mirrors around your tank, hopefully the bully will start attacking it’s own reflection rather than constantly picking on your other fish.