To Keep Big Marine Fish

How to keep big marine fish

Big and bad? Or big and beautiful? Last month we looked at some of the bigger marines and looked at possible candidates for a marine aquarium as well as looking at unsuitable species and what makes certain fish inappropriate for the aquarium. This month, we are looking at what issues you need to deal with if you are keeping one of the larger species, such as some puffers and triggers.

Do your homework

The most important starting point is research. Don’t buy on impulse. Most marine fish are imported and offered for sale at a fraction of their eventual size. This helps keep costs down, as it is possible to ship several small specimens for the same cost as one large specimen.

There is also a conservation issue, as if large fish were frequently exported this would leave fewer breeding adult fish to repopulate the reef.

Many small fish will be eaten before they mature, so it makes sense, from a financial and environmental viewpoint to buy only young fish.

However, this situation makes it entirely possible for fish keepers to walk into a shop and purchase a 5cm/2″ fish not realising it may outgrow their aquarium.

While I, like most retailers, freely give information to customers about their potential purchase, there is always the odd one that may slip through the net. (No pun intended). That said, very few customers quiz shop staff before making a purchase.

Buy a big tank!

It seems obvious, but a big tank is vital. It is generally accepted that the maximum stocking level for a fish only marine aquarium is 2.5cm/1″ of fish per 9 l./2 gal. of water capacity.

This works fine when the fish are only small, as there is little difference in weight and consequently the waste produced between a 5cm/2″ fish and a 10cm/4″ fish. As fish become larger the amount excreted increases disproportionately to its size.

For example a 10cm/4″ fish is likely to weigh about 25-30 grams, a fish of 30cm/12″ is likely to weigh nearer 500 grams, so will produce a massive amount of waste, even when compared with several smaller fish.

This brings the whole idea of currently accepted stocking suggestions into doubt. It is my opinion, that a stocking level based on fish biomass would be a more accurate way of determining sensible stocking levels. After all this is how it is calculated in commercial situations.

To stand any chance of having a sensible ratio of fish to water a decent-sized aquarium will be required. What do we mean by a decent-sized aquarium? Well, this really depends upon the adult size of fish you intend to keep.

As a rough guide aim for an aquarium four times longer than the fishes length and the width should be at least one and a half times the fish’s length. This means a 30cm/12″ fish will require an aquarium at least 122cm/48″ long and 45cm/18″ wide. Even this will be quite cramped for most species, after all we wouldn’t dream of keeping a 5cm/2″ fish in a 20cm/8″ by 7.5cm/3″ aquarium! I hope not, anyway.

Big fish, big filter

As we are now aware, the waste excreted by bigger fish is huge even when compared with several smaller fish. This will call for an equally large filtration system to deal with this waste.

We often hear retailers going on about how important a protein skimmer is, but this is not just sales talk. I look after a 1702 l./375 gal. aquarium fitted with two Prizm Pro skimmers each with a stated capacity of 1203 l./265 gal.

Every week these remove a couple of litres of almost black liquid with a dreadful smell. It is stocked lightly with larger wrasse, tangs and a 25cm/10″ puffer.

By comparison the skimmer on our 9080 l./2000 gal. invert system removes less than a litre in the same period. A substantially oversized skimmer is more important on a fish only system than a reef aquarium.

For mechanical and biological filtration large external filters work well if filled with a high surface area media. But these may only filter half the water volume recommended by the manufacturer, so for a 454 l./100 gal. aquarium you will probably need a filter for 908 l./200 gal.

Remember we are dealing with fish that produce many times more waste than the average fish. The big advantage of externals is that they can be filled with media of your choice and easily fitted to almost any aquarium without drilling.

As fish get larger they require more oxygen, and produce more waste. More bacteria are needed to deal with this waste, but in turn consume more oxygen. This can result in water being de-oxygenated in aquariums with a heavy loading.

Trickle filters avoid this scenario by taking oxygen directly from the atmosphere. This results in an increased dissolved oxygen levels as well as increased filter efficiency.

Many also have a pre-filter that can be cleaned daily, this reduces loading by removing waste before is has chance to breakdown.

Most trickle filters are mounted externally below the aquarium in a smaller tank called a sump. These are ideal for aquarium containing larger fish as heaters and other easily damaged equipment can be installed here, out of harm’s way.

Turn on the light

An ultraviolet steriliser, or UV, is a worthwhile investment for any marine system. They prevent the outbreak of many diseases, and may have a small effect on water clarity and algae problems. They are especially important with respect to large wrasse, for example Harlequin tusks as well as sharks and rays which all tolerate copper-based medications poorly, if at all.

While metal halides look great on any aquarium, they are not needed for a fish only system. Adequate illumination can be provided by one or two fluorescent lamps designed for marine use. These should be protected by a set of cover glasses, as large fish are capable of soaking electrics and the surrounding area, especially at feeding time.

Any bare tank looks pretty boring. Adding decoration changes a tank into an aquarium. Even with large fish it is possible to have something other than a sparse aquarium. The rockwork used should be more of a permanent feature, rather than an afterthought.

Ideally all rock should be in place prior to filling with water. Rocks used should be large enough to stay in place if knocked by large fish. If necessary these can be a held with one of the epoxies available. Leave plenty of space at the front for swimming.

Rising costs

Feeding larger than average fish also creates another problem – namely cost. Large fish can eat huge amounts of food, so buying small packs of frozen food from your retailer proves expensive.

I kept a very large Porcupine puffer for some time, and it ate one whole 80g pack of food at each sitting. A visit to your local supermarket may be productive; they often sell frozen prawns in large bags for just a few pounds. These are ideal for larger marine fish. Avoid oily fish as these will really tax your filtration system.

Feeding big fish results in the inevitable rise of nitrate and phosphate, these can increase at alarming rates. Most large hardy fish can tolerate elevated nitrate levels, although this is no excuse to neglect maintenance.

High nitrate will cause the fish to be more susceptible to disease, as well as dulling colours. Frequent large water changes will be essential to keep nitrate from rising to dangerous levels.

The denitrators (nitrate filters) on the market may assist in stretching out water changes, but should not be relied on too heavily. Most need frequent monitoring and adjustment to work efficiently.

Phosphate can be a major factor in algae problems, even low levels sometimes cause an unsightly outbreak. You can’t use hermit crabs or snails to control algae in a tank containing big predators, since they’ll probably eat them.

As an alternative, large tangs will help keep down algae, although they cannot expect to eat all the algae growing due to poor water quality. Even with frequent large-scale water changes phosphate remover will be a necessity to prevent algae.

Hopefully, this should go some way to helping your large marine fish venture a success. Remember to research before you buy and you will avoid the problems posed by unsuitable aquarium inhabitants or inadequate equipment.

Big fish story

About fifteen years ago when I had just started in the aquatic trade we had a very pretty yellow and black grouper about 5cm/2″ in length.

I had tried to identify it but without success. One of my responsibilities was to feed the marine fish, this fish fed like no other, in fact it ate so much it laid on it’s back for a few hours! I was terrified I had killed it, and was going to get into trouble after all this 5cm/2″ fish was 55, about a week’s wages for me at the time.

Anyway the grouper survived, and I learned to feed it much less, it was sold after six weeks and had grown to 20cm/8″ in this time.

One night I was flicking through my collection of fish identification guides and come across information regarding the grouper we had sold recently.

It turned out to be a Queensland grouper, Epinephelus lanceolatus, and had an adult size of 3m/10′!

One day we received a phone call from a customer who had a grouper that had outgrown his aquarium. When he brought it into the shop, it turned out to be the Queensland grouper we had sold a few months earlier.

It had grown! It was now a 60cm/24″ monster that must have weighed 5kg, with an appetite to match. Luckily Queensland groupers are very rare; this makes them sought after by public aquariums where it was eventually re-homed.

Unless your large marine fish is exceptionally rare the chances are most aquatic shops and public aquariums will shun you like you have the plague!
If you can’t house it when it reaches adult size don’t buy it.

This article was first published in the March 2003 issue of Practical Fishkeeping magazine.


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