5 Cardinal Sins Of A Marine Aquarist

I thought I’d jot down what I consider to be the five major sins of a marine aquarist, be that for a reef tank or a fish only tank. They are not in any particular order. The list is not just aimed at beginners to the hobby who are just starting a saltwater aquarium but at aquarists of any level.


This is self explanatory, and is probably for the most part the domain of the new aquarist. Overfeeding occurs so easily, with those beautiful fish swimming close to the front glass ‘begging’ for food. Overfeeding can occur with the best of intentions, the aquarist being concerned that the fish (and whatever else) are sufficiently fed. Flake food (for example) is concentrated and rich, and pollutes the water very easily. That is the problem, pollution. Eventually, even with routine water changes, water quality reduces. Food is a major source of nitrate and phosphate, both of which are nutrients for nuisance algae.

The solution to overfeeding is simply don’t do it. Observe the livestock. Are they well fed? Remember, they will obtain some food from the reef/decorations. Discipline and observation are the essentials.

Failure to Test the Seawater

It is repeated over and over again by every authority that seawater quality is the number one on the aquarists list of ‘must haves’. The seawater in the aquarium is nearly everything to livestock, they are touching it constantly and rely on it for oxygen and other essentials. Food alone, no matter how high the quality, will not sustain them entirely. As soon as seawater is in the aquarium with livestock, it begins to deteriorate. The rate of deterioration can be slowed down by, in particular, the employment of a high quality protein skimmer. Other aids can be the reef itself (live rock), a deep sand bed, a calcium reactor (some of these inject more than calcium) etc. Nevertheless, seawater quality deterioration is there. The fish only aquarist can do fewer tests than the reef aquarist, but whatever the livestock, testing must be done. Once experience has been gained then testing frequency can be reduced. However, the aquarist must always be aware of the state of the seawater. We would not be happy walking around in and breathing polluted air.

Routine water changes should be completed, using reverse osmosis water for the mix. At the start, 10% (of the systems net gallonage) should be changed. Again, this can be reduced when experience is gained, either by lowering the percentage or increasing the period between changes, and also subject to a light or heavy bio-load. In some cases the percentage may need to be raised to maintain high quality seawater, but if this is the case an examination of filtration, feeding and stocking should be undertaken. Are the filters functioning correctly, and are they sufficiently sized? Is the system overstocked? Are the livestock being overfed?

Failure to Renew the Light Bulbs

By light bulbs I am referring to metal halide bulbs or fluorescent tubes. Lighting needs are more appropriate to the reef aquarist. Lighting is a close second to water quality. A large number of corals need lighting of the correct intensity and spectrum to flourish. Light bulbs reduce in efficiency as time passes. This efficiency loss is because of light intensity reduction and also spectrum shift. The aquarist pays quite an amount for bulbs, so they should at least perform as designed. Changing the bulbs at regular intervals achieves this. Fluorescent tubes should be changed at six months to one year. Halides should be changed at one year to two years. Look at what the manufacturer states, and pay heed to what other aquarists who use the same make and type of bulb say.

If light requiring corals are not flourishing, and seawater quality, including seawater movement, is known to be fault free, consider the lighting. Is there a problem? Do the bulb(s) need changing?

Incompatible Stocking

The aquarist may have very high seawater quality and lighting may be fault free, but some of the fish (for example) may be sickly, feeding poorly and obviously unhappy. When on the wild reef fish have few objectives in their lives, but the ones they do have are :

  1. to breed
  2. to avoid confrontation
  3. to find enough food
  4. to avoid becoming food

So if a small fish has gone missing, has a predator (example: a lionfish Pterois volitions) been introduced? Don’t smile, you’d be surprised at what is purchased sometimes on impulse. Is a fish showing ragged fins, poor colours, hovering in a corner, or hiding in rockwork, hardly coming out even to feed? Is it being harassed by bold and aggressive fishes? It is not only inter-fish problems that must be avoided. There are fish that would love to eat that shrimp. There are shrimps that would love to eat that starfish.

Research what is being put into the aquarium. Ensure full compatibility. It is cruel to fail to do this. Nowadays there are many quality books available, and the internet where information is freely available.


Large or small, every aquarium system has its limit in the amount of livestock that can be kept. Keeping too much livestock, in particular fish, increases the bio-load that has to be dealt with. The biological support of the aquarium will increase over time as the bio-load increases – up to a point. Then it is unable to deal with the wastes and disaster is just around the corner. The aquarium inhabitants face death by poisoning. The need for large seawater changes is going to increase. Even then, disaster is close. The aquarist has put all his/her efforts in jeopardy, including seawater quality and even all aquarium life.

Quite apart from the dangers of reduced seawater quality and failing biological support, there is the question of the space needs of the inhabitants themselves. Fish etc need to feel secure in order to prosper and be healthy, which means they need to be able to find a hide hole during the dark hours and in daylight hours have a hole to disappear into. If the aquarium is overstocked holes are going to be at a premium and aggression and fights could ensue. On the wild reef it is life to have a secure hole and the instinctive need does not disappear in an aquarium.

Many aquarists, particularly those with reef aquariums, don’t stock to the theoretical capacity. They under stock knowing that seawater quality will be better, and the corals and fish will be all the better for it. Avoid the temptation of ‘just one more fish’.

I hope that this has been useful to you and would love to hear any comments you have about your experiences.

  1. Nice list. Let me sneak in:

    5.1 – Impatience
    5.2 – Inadequate flow
    5.3 – Insufficient research

  2. Absolutely right there Andy. Patience is the best virtue the marine aquarist can possess. Adequate seawater flow is part of seawater quality for me, but is a separate item for most, so right again. Of course, insufficient research is probably what leads to overstocking and certainly also leads to incompatible stocking.

  3. I agree to Andy – 5.1 – Impatience and 5.2 Insufficient research.

  4. Loving the info on this website , you have done great job on the blog posts.

  5. Hello Elmo thanks for the comment and compliment.

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