Stocking a Marine Aquarium

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So you've finally decided to take that plunge into the wonderful world of marine aquariums. You may have been keeping freshwater for quite some time now and all the knowledge you have picked up from that is going to come into play here. However, this one area that your knowledge is not going to translate very well, and that is in the area of stocking. A marine aquarium cannot be stocked as heavily as a freshwater.

First reason you can't stock marine aquariums as heavily as freshwater tanks is due to the sensitivity of the organisms. Marine fish are much more sensitive to nitrates than most freshwater species. Many species of marine fish are intolerant of nitrate levels that exceed 5-10ppm. Now a second consideration comes into play here and that is the choice between a Reef setup and a Fish Only With Live Rock (FOWLR) setup. While the fish may be able to tolerate levels up to 10ppm, most marine invertebrates will not tolerate a level that exceeds 5ppm, and if you are going to keep an anemone the nitrates need to be no more than 1-3ppm. Because of that high demand for very low nitrates, a Reef setup must be stocked lighter than a FOWLR setup. In a FOWLR setup you are more likely to be keeping larger fish that are not reef-safe. These larger fish are often much heavier waste producers and thus will foul their water much quicker. For this reason heavy waste producing fish must be kept in larger tanks. Here's a scenario where a beginner may get tripped up; they buy a 125 gallon tank. They decide that they really love puffers so they go out and purchase a nice Spiny Puffer – Diodon holocanthus aka. Porcupine puffer. Then a few weeks later they discover Triggerfish and decide to add a very attractive Clown Trigger – Blistoides conspicillum. Then finally they decide they want something a little on the predatory side and add the ever popular Volitans Lionfish -  Pterois volitans. Now, all three of the above mentioned fish would be suitable choices for a 125 gallon FOWLR setup. However, believe it or not, with just 3 fish in that tank, the beginner has unwittingly heavily overstocked their tank. All three of these fish are known heavy waste producers. The Lionfish is a predator and thus it is a given that it will create a large amount of waste. Puffers are messy eaters as are Triggers. The beginning marine hobbyist is set to spend a lot of time changing a lot of water. So with that scenario, it becomes evident that overstocking is not a difficult thing to do. So you might be thinking, “no problem, I'll just add more  Live Rock to get a larger bio-filter. If bioload was the only consideration then you would be correct, however it is not and things get a bit more complex.

Lionfish

The second thing to consider is the aggression level of marine fish. A large majority of the fish we keep in our tanks are native to the world's tropical reefs. The reef environment is definitely not a peaceful one by any stretch of the imagination. A day on the reef is a constant battle for survival for may fish. When we think of classifying the aggression levels of our freshwater fish we usually think in terms of peaceful, semi-aggressive, and aggressive, however, in a marine environment there is no peaceful, just varying degrees of aggressive. In the marine environment the fish are generally classified as semi-aggressive, aggressive, or predatory. When you see the term peaceful applied to marine fish it is a very relative term. Relative to the rest of the reef inhabitants, the Ocellaris Clownfish – Amphiprion ocellaris is very peaceful, however, even that cute little fish affectionately called “Nemo” by so many kids will get aggressive in the right situations. This aggression in marine fish generally comes from being very territorial. Many species of fish have their own little cave or crevice that they inhabit and they will defend it tenaciously, and size rarely matters. These caves are chosen based on availability of food and protection provided. On the reef there is lots of space and plenty of room for each individual fish to have it's own territory, however once they are moved into the home aquarium everything changes. The confines of a tank are much smaller than the reef and more often than not there is insufficient room for some of these fish to have their own territory, so the entire tank becomes their territory. On the reefs you will often find groups of  Royal Grammas – Gramma loredo living in a small cave, however you are unlikely to succeed when attempting to keep more than one in a tank under 180 gallons, even with plenty of hiding spaces they are likely to spot each other and eventually, only one specimen will be left.

So, when considering what to keep in that new marine tank, keep these two things in mind, keep your nutrient levels low and hiding spots plentiful. A natural reef with thousands and thousands of fish is a beautiful thing, but remember, you are only dealing with a small portion of that reef. The natural reef has the whole ocean as it's filter, your tank doesn't. The natural reef has a vast area with millions and millions of hiding spots, your tank doesn't. So, keep it light on the stock and enjoy the wonderful world of marine aquariums.