‘Big’ To The Marine Aquarist

Big Fish

Most marine aquarists keep mixed reef aquariums, some of them are very small ‘nano’ systems and others could be six footers, large in the hobby world. There are a few who own aquariums even larger. Other aquarists have fish only systems that could be the same in size range to the captive reefs.

Mention ‘big’ and the reef aquarist thinks of a system larger than the one he/she already has, a huge affair maybe that looks like part of the Pacific Ocean has been transported for the aquarist’s personal use. Or perhaps thoughts turn to a more usually dimensioned system but still larger than the one already owned. I suppose its human nature to think this way.

There’s another meaning to ‘big’ though and this time it doesn’t refer to the aquarium size but to the livestock in the aquarium, specifically big fish. There are those who find keeping large fish fascinating and it is a side of the hobby that is often overlooked, as said most aquarists think of mixed reefs and small fish only systems.

Keeping big fish is a discipline in itself. Considering the diversity of size on the wild reef there is a fair choice, though it could be the larger fish are not so plentiful in local fish shops as demand is not so high. We’re not talking of the very large reef fish but those that are considerably larger than those usually kept in a reef or even a fish only aquarium.

Keeping large fish requires careful thought – there will not be as many for a start. There could be one or perhaps two or three. The stocking guidelines apply, it should be particularly remembered that the net seawater gallonage dictates the fish size/numbers that can be accommodated. So any rockwork that supplies a home for fish will reduce the gallonage. A sump will increase seawater gallonage (and quality) but should not be included in the stocking calculations as to do so could lead to too high stocking and subsequent aggression. The size of aquarium is obviously very important and a very large one, or the largest practically useful, should be obtained so that accommodation will not be excessively restricted. The guideline for the aquarium size is that its length should be four times that of the biggest fully grown fish and the depth and width at least twice the length of the biggest fish. The guidelines on aquarium placement also apply.

When considering the size of aquarium it is best to research the fish that are desired first. Fish grow and need more space, so knowing their eventual likely size will either cause a search for more suitable fish because the needed aquarium is too large, or confirm the intended size is adequate. The fish in the shop could be small, but to what size will it grow? Some grow quickly!

Lighting is not the consideration it is with a reef system. What is required is sufficient light for the fish to see and be seen. This is easily met by two aquarium length fluorescent tubes. They can be selected to enhance the fish colours; two that are usually good for this are ‘marine white’ and ‘actinic blue’. In addition, it is good to have a ‘dawn’ and ‘dusk’ sequence which is achieved with electric timers. One timer turns the blue tube on and off say one half hour before the white tube comes on and the same period after the white tube goes off. Flooding an aquarium with sudden light or plunging it into sudden darkness is not good management.

During the research of the fish temperament should be carefully considered. The fish could well be an interesting and beautiful one, but also very aggressive and/or a predator. Suitable tank mates have obviously to be found for such a fish or trouble will not be long in coming. Fish that grow large could be purchased at a considerably lesser size, but it is still incorrect to house the fish with fish that have a full grown smaller size. Fish in good conditions could and hopefully will have a long life – but not if there is a growing aggressive fish or predator in the aquarium.

Research should also cover the suggested captive diet of the fish, so the aquarist knows that he/she can cope with the demand – is it available locally and is it able to be stored in the home ready for use?

Big fish eat big and the filtration system has to cope with it. The filtration is that operated by the bacteria of course (the bio-filtration) which must be able to handle and convert the generated waste. In addition big fish are generally messy and mechanical filtration is usually required, the canister filter is probably the best choice. The canister filter should be large enough to cope with the amount of seawater present and one a little oversized will not go amiss. Some aquarists use two canister filters of appropriate size, one for purely mechanical filtration and one for bio-filtration. The bio-filtration unit will also have mechanical filtration to protect the bio area. With both filters, particularly the bio one, it is important to regularly clean the mechanical media and replace it if it is suspect.

Regular maintenance applies to the big fish aquarium as it does to any other. There is a need for regular seawater changes. The commencing guideline is 10% of the net system volume per week but the aquarist should adjust this if seawater quality is below that required. Cleanliness of the aquarium needs watching and seawater siphoned out on a routine change gives the opportunity to remove detritus – big fish as mentioned could be ‘messy’.

The words ‘seawater quality’ bring up an important piece of equipment that most certainly should not be missing on a big fish aquarium, and that is the protein skimmer. A model should be selected that is fully capable of dealing with the net system seawater volume, including any sump. An old guideline suggested that when purchasing a protein skimmer it should be capable of dealing with twice the seawater volume present, and if there is any doubt it is a good guideline to follow.

Again dealing with seawater quality there are five test kits that are required, ammonia (there should never be any reading present*), nitrite (there should never be any reading present*), and nitrate (which is best kept as low as possible but as a guideline should not rise above 30ppm, better 20ppm. Nitrate levels are usually kept in check by routine seawater changes). *Ammonia and nitrite will make an appearance in any maturation period required with a bio-filter.

Nitrate and the fact that big fish are big eaters have already been mentioned. If the aquarist finds that nitrate is rising despite reasonably sized routine seawater changes, then another piece of equipment could be considered and this is the denitrator. Basically the denitrator is a home for bacteria. The environment is maintained at a very low oxygen level and the bacteria require oxygen, therefore to obtain it they extract it from the nitrate which breaks this down. There are mainly two types, both commercially available. One is based on sulphur and requires no attention except to ensure that the slow flow of seawater through it is maintained. The other type requires feeding with supplied nutrients. Personally I prefer the sulphur type but it is a matter of personal choice. It could well be that a denitrator is not required.

It was stated in a previous paragraph that five test kits are needed and three have been mentioned. The other two are first a pH test. This is to check the alkalinity of the seawater. The guideline range is 8.1 to 8.4. Whichever it is, it should be stable. Routine seawater changes should assist with this. The final test is not really a kit as such, it is an instrument named a hydrometer. This checks the specific gravity (SG) of the seawater; the guideline suggests a range of 1.022 to 1.026. With a fish only aquarium it would be unusual for the top end of the SG range to be used. Using an SG of 1.022 is reported to be beneficial to fish as it is more detrimental to the well being of certain parasites. The SG chosen should be stable.

All the tests should be completed once a week; they are straightforward and not particularly time consuming.

Keeping big fish means the aquarist will not have so many to look at. However, it follows that there is more of each fish to see! In addition, it could be argued that the visual impact of the fish is greater. Many have beautiful colours. It is known that some of these bigger fish become real pets with individuality and personality and aquarists and their families become very attached to them.

  1. Great post John!

    You’re always coming up with awesome and in-depth pieces. Your work is much appreciated! I love the big fish too and would be great to have a few down the road. Right now I’m working on a 75g mixed reef focusing on a smaller variety of fish with the largest being a Tomini Bristletooth Tang. THe big ones definitely do get the “oooh” factor going.


    Brian Blanks last blog post..April: 30 Posts in 30 Days

  2. Thanks for that Brian.

    Hope the 75 gal comes together well and is a success. I’m sure it will be!

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