5 Reasons to Understock Your Marine Aquarium

understock-aquarium1Over the many years I’ve kept marine aquariums, guests in my home have more than occasionally noted the relatively low number of specimens, particularly fish, in my tanks. You could say it’s been a hallmark of my fishkeeping career to understock my aquariums, sometimes to the point that they look rather sparse. But there’s a method to the madness of my modest stocking levels.

Here are five benefits that I derive from stocking sparingly—and you can too:

1) Fewer compatibility issues

While I’ve seen my share of piscine pugilism, most of my aquariums have been fairly tranquil. This is due in part to careful specimen selection and introducing species in the proper order (least aggressive to most aggressive), but I’m convinced that giving the fish plenty of elbow (fin?) room so they can stay out of each other’s business has played a big role in promoting the peace as well.

2) Decreased demand on filtration

In a lightly stocked aquarium, the biofilter (if well established), protein skimmer, and any chemical filtrants used can easily keep pace with the nitrogenous wastes and dissolved organic compounds, providing a good margin of error against ammonia spikes and other water-quality issues.

3) Reduced maintenance

Having a lower level of dissolved pollutants translates into slower algal growth, slower accumulation of nitrate, etc. As a result, it generally takes less “elbow grease” to keep a lightly stocked aquarium in good health and looking its best. I’m not saying you can start to skip water changes, but if you get really busy for a period, waiting just a bit longer in between changes usually won’t have major negative ramifications in an understocked tank.

4) Room to grow

Most of the fish specimens sold at your local fish store are juveniles, which means that, over time, they’re going to get bigger—in some cases much bigger. A tank housing a relatively small number of juvenile fish may seem empty at first but then look just about right when the specimens gain a little size. Of course, this same point applies to corals and other inverts—little colonies grow into bigger ones, taking up more space physically and visually.

5) Room to add one more

You never know when that dream fish or hard-to-find coral that you assumed you’d never see in real life will suddenly become available. If/when that happens, you’re going to wish you had the space in your tank to accommodate it (assuming it’s otherwise compatible with all your current livestock). Why not build in that possibility right from the start?

Photo credit: Lance Andrewes


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. The title of this blog should be 5 reasons to “purpossly” understock your marine aquarium as in time any “free” spaces will be filled with livestock. Unintended livestock. We should of course keep our purchased livestock lower for the reasons mentioned but we often fail to realize that eventually, if the tank is healthy, that multitudes of creatures will fill those voids and although they are tiny, when you add them up, they can contribute more to the bio load than the creatures that cost us a large sum of money. My tank for instance is absolutely loaded with tiny tube worms that fill every nook anc cranny. I couldn’t count the bristle worms, some of them could be related to Godzilla. Amphipods and copepods vie for real estate rights and the tiny snails, shelled and un shelled could pass for a bag of pistachios. These things on their own are miniscule but if you add them up, they can number more than the number of Geeks at a Lady GaGa concert.

    • Jeff Kurtz says

      I know what you mean, Paul. I just caught a glimpse of a bristleworm in my reef tank that you could put a saddle on!

  2. Greg Heifner says

    I agree as well. Stress reduction is a huge part of keeping fish like tangs and others. Treat you reef as you would planting a garden, it needs room to grow,

  3. I never over stock my aquarium. I have 2 seahorses that have a 180 l tank all by themselves. And my larger reef aquarium mostly has corals. Makes looking after the aquarium so much easier.

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