The Salty Side: Saltwater Aquariums for Beginners

New Fish Are Not Eating

I’m sure most of us can relate.  We’re excited about having bought an awesome new saltwater fish for our reef aquarium.  But despite looking healthy, it just won’t eat.  In this article, I will look at ways in which we can get to the bottom of why your new fish are not be eating and the different solutions. 

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Why is my new fish not eating?

There can be several reasons why your new saltwater fish may not be eating.  The most common are:

High ammonia levels

I would recommend following the steps below to try and get your new fish to eat.  If none of them work then it may be a specific disease/illness you are dealing with and more research will be needed as to how to treat effectively.  It is unfortunate sometimes though that no matter how hard we try, we can still end up losing stock.  It sometimes helps to change suppliers as I have found that fish from one local store have a far higher survival rate than those from another in the area. 

How long can saltwater fish go without eating?

Some can go for as long as a week.  But if a fish is suffering with an illness or disease that has been left untreated then it may not last a whole week.  When buying new fish, these can go unnoticed and with the stress of being caught, the journey home and being placed in a whole new environment, it may just be the tipping point and prove fatal.  

What to do if your fish is not eating

Well hopefully, you will already know that quarantining new fish is essential.  It gives your new fish the best possible chance of settling in well into your main tank.  If you don’t quarantine then I highly suggest you take a look at our guide on setting up a quarantine tank for new fish. 

If your new fish has gone straight into a new tank and is not eating after a few days, I would set up a hospital tank.  You will then be able to treat it without being concerned that the medication with interfere or harm other fish and especially invertebrates and coral. 

Another reason this can be beneficial is that you are removing the stresses that come with being the ‘new fish’ in a tank full of territorial fish and their sometimes aggressive behaviour.  That being said, you will be stressing it out even further and even once in a hospital tank, it may have proven to be too much for the fish to handle and nothing you try will help it survive. 

BUT, we have to try!  So here are a few ways in which to help your new saltwater fish start eating:

1. Feeding with live food

Live food is an important part of the diet of a marine fish and a well-balanced diet means healthier fish.   Live food can also trigger a feeding response in fish that are reluctant to eat.  These can be bought either online or at your local fish shop (lfs).  

Examples of live food are brine shrimp, mysis, copepod and black worm.  If you cannot get hold of any, you should at least be able to buy brine shrimp eggs and hatch them yourself within 24 hours.  You can see how I hatch brine shrimp in this article.  It’s super easy!

Although these can be great to get a fish starting to eat again, brine shrimp should not be relied upon as a sole source of nourishment.  They have very little nutritional value to saltwater fish. 

Some fish are also just fussy!  If they’ve been fed with a certain food, whether it be live, frozen or flaked, this is what they may be used to.  Try varying their diet and see if that helps.

2. Feeding with bivalves

Oysters, clams, muscles and scallops all fall under the bivalve category.  They are usually frozen and can be found at most fish stores or online.  

These are worth trying as this is what a fish would naturally eat in the ocean.

3. Check for increased ammonia

A quick and easy solution would be to do a 50% water change.  High levels of ammonia is detrimental to fish which is why a tank needs to be fully cycled before adding fish.  The same goes for a quarantine tank.

Briefly, there are two ways in which to cycle a quarantine tank.  One requires a sponge filter, or piece of rock, that needs to be ‘seeded’ in the main tank at least 2 weeks before a new fish is added.  OR, you can purchase live nitrifying bacteria, which will break down the ammonia within days.  (This article explains more about tank cycling.)

In order to know for sure whether your levels are too high, you will need an ammonia test kit.  

4. Treat for Parasites

If you have tried the above and your new fish is still not eating, I would recommend Prazi Pro.  It successfully treats tapeworm, flatworm, flukes and turbellarians.  Once started, it should be repeated every 6 or 7 days. 

Chelated/Ionic Copper is also used to treat parasites such as ich.  This is where having a fish in quarantine has the biggest advantage.  Copper is toxic to invertebrates (snails, hermit crabs etc.) and beneficial bacteria so if your new fish needs treating, you can safely treat it in the quarantine tank without affecting the other inhabitants of your main tank. 

Please note that these two medications should not be used in conjunction with each other or any other medication.  Neither should they be used whilst practising hyposalinity.  This will only lead to a concentration of the medication which could end up proving fatal.  

Below is an informative video on how to medicate fish with copper.  

So there you have it!

You can take the proactive approach and dose all new fish with Prazi Pro initially.  Unfortunately you cannot always see what parasites a fish may have and sometimes it’s better to just assume it has parasites.  It won’t do them any harm and might actually end up saving them. 


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