The fish only system is probably not as popular as a captive reef but there are plenty of aquarists who maintain one. There are advantages to it as well.
The fish only aquarium can house fish that would not be welcome on a captive reef because they are predatory on other fish and/or shrimps, or find corals good to munch on. Habits like these are certainly not going to increase their standing in the reef aquarium popularity stakes. There can also be a higher fish load. Though seawater quality remains a high priority it is not quite as demanding as with the reef aquarium. Further tests could be made but basically pH, phosphate, nitrate, and SG (specific gravity) are those that should receive attention (assuming that maturation has taken place and the system is stable with a functional bio-filter). Lighting is also not as demanding, the basic requirement is for the fish to be able to see and be seen.
Despite the lower demand for high quality seawater, efforts should always be made to achieve the highest possible. The nitrate guideline for fish only is 30ppm (parts per million) or less, phosphate preferably undetectable but no more than 0.03ppm, SG 1.022 to 1.025 and pH 8.1 to 8.4.
The fish only aquarium is not completely plain sailing though. There could be problems. These are also possible with a reef system but less likely. The following is not in any particular order of priority.
Oxygen is very important to fish as it is to us. However, there is around twenty times more oxygen in the air we breathe than in warm seawater. The usual level of oxygen in warm seawater is around 6 to 8ppm. Just a few parts per million! The warmer the seawater is the less oxygen, so in a heat wave where there isn’t any efficient cooling apparatus the oxygen will reduce. So it is clear that maximum oxygen intake is essential. Seawater circulation should be efficient, as this brings the seawater to air/water interfaces such as the aquarium seawater surface. It is at these interfaces that oxygen intake takes place. Making use of a sump is a good idea, not only does it increase the net gallonage, it permits seawater to run over a weir or similar, which effectively removes any scum that could accumulate on the seawater surface. This scum can reduce the efficiency of the seawater surface interface.
Nitrate is well known to all marine aquarists and is to be avoided as far as possible. It is a known nuisance algae nutrient which is a good reason to keep it down. Generally fish are able to tolerate nitrate better than corals, though there are more sensitive fish. Nitrate needs to be minimized anyway to enhance seawater quality.
Nitrate gets into the seawater as part of the Nitrogen Cycle, following on from ammonia and nitrite. Once it is in the seawater, it stays there unless there are nitrate reducing agents present, such as live rock (which is able to deal with nitrate but only within reasonable limits), denitrators etc. In a fully stocked fish only aquarium it is likely that nitrate will be present anyway.
The first action that controls nitrate is maintenance, namely routine seawater changes. The guideline, which can be adjusted by the aquarist as experience grows, is 10% of the total system net gallonage per week. In addition, when old seawater is being siphoned out any detritus should also be removed.
An efficient protein skimmer is a must. The device should be properly sized so that it can easily handle the total gallonage of the system. The protein skimmer removes dissolved organic substances (Docs) completely out of the seawater. This is very helpful, as it means the bio-filtration will not have to deal with it, so the load on the filtration is reduced. Further, if the bio-filtration is not dealing with it, it will not lead to additional nitrates.
The main source of nitrate is from feeding. The fish will naturally process the food of course and it will eventually become nitrate. Any additional food that is not eaten will also join the nitrogen cycle and become nitrate, as will rotting algae etc. Feeding is very often overdone, particularly by beginners, this is very detrimental. There are usually more fish in a fish only system, size for size, than a reef, so more food is required anyway. Feeding the livestock is very enjoyable and the aquarist is concerned that they get enough, so it is more than easy to overfeed. It is a discipline that needs to be learned however, as overfeeding is just the opposite of ‘good’.
Phosphate as already mentioned should be at a very low level. Most phosphate in the seawater gets there from the food fed to the fish, so again it is very necessary to discipline feeding. As with nitrate, if there is a phosphate problem the feeding method must be critically examined.
Overstocking must be the worst problem, simply because it leads to others. The aquarist could have been aware of the guidelines for stocking and adhered to them, but still the aquarium becomes overstocked. This is because when the fish were originally purchased they were small. Many fish for sale in retailers are small. However they all, to differing extents of course, have the potential to grow, and they will. What started out as a correctly stocked aquarium becomes badly overstocked. The potential for trouble looms larger.
As the fish grow they demand more food which the aquarist supplies. This leads to more waste for the nitrogen cycle to deal with, which in turn leads to more nitrates. Unless the aquarist increases the seawater routine change amount to combat the rise, the nitrate is likely to slowly increase, which decreases the quality of the seawater. Filtration such as live rock continues to deal with some nitrates but the increased amount in the seawater could be beyond its capabilities – the amount of bio-filtration material was calculated for a correctly stocked aquarium. This means, particularly if deterioration is permitted to continue, that the fish become more likely to suffer health problems.
Then there is the all important oxygen. The larger the fish get the more oxygen they consume. It could well be, hopefully, that there continues to be enough oxygen for the fish. What happens though if the temperature rises, which reduces the oxygen content of the seawater? The demand for oxygen by the fish doesn’t reduce. The fish could start to gasp near the seawater surface. If this situation is reached it is serious.
It is worse than that! The fish are consuming oxygen from a dwindling supply; they could start to gasp as said. There is another oxygen hungry item in the aquarium, one that is essential to its well being. This is the bio-filter, where all the hardworking bacteria reside. These bacteria convert the toxics ammonia and nitrite into nitrate and it is essential they function properly. If they don’t because of reduced oxygen levels then a serious situation could become dire.
Overstocking could lead to a further problem and this is aggression. To a varying extent fish need space and also a place to call home. If they haven’t a secure place to hide overnight then they could attempt to obtain one by taking over from another fish. The other fish is not likely to be happy with that. Aggression produces winners and losers, and if a fish is not secure and is harassed there is a danger to its health. Sometimes aggression doesn’t occur, such as in a dealer’s aquarium, as there isn’t anywhere to hide anyway. In a home aquarium there should be, the aquascaping should deal with it.
Overstocking is easily avoided by simple research. When fishes are being chosen their likely final size should be considered. This will reduce the fish numbers that could be initially housed. Another way, one that is very second rate in my opinion, is to stock for the purchase size and when growth is a problem either return the fish to the dealer on exchange or swap with another aquarist. This method is not so good because it could be the dealer is not interested in larger fish as there isn’t a market for them, and other aquarists could have a full stock anyway and not want more. Better to consider potential growth and stock on that.
Incidentally, when calculating stocking levels ignore any sump, this is because the fish are in the display aquarium and the sump area is not available to them. If the sump were to be included the potential for overcrowding would increase. The gallonage in the sump will also assist with overall seawater quality.
If the aquarist stocks the aquarium properly he/she will know that the oxygen supply is unlikely to cause problems (this does not mean that in extra warm weather cooling of the seawater is not required). It will also be known that the bio-filtration, essential to the well being of the fish, will be able to cope. If routine seawater changes are done, and they are of a suitable amount, nitrates and phosphates are unlikely to be a problem, or at least the problem potential is reduced, also reducing the potential for nuisance algae. If the aquarist feeds with discipline there will still be pleasure in the process, but the pressure on the bio-filtration will be lessened and the production of nitrates and phosphates reduced.
A well set up and cared for fish only aquarium can be colourful and fascinating. Whether there are many small fish or one or two larger species, they’ll be in fine shape.