Signs You May Need to Lower Your Reef Tank’s Bioload

The maximum level of acceptable bioload is unique to each system based on a variety of factors

The maximum level of acceptable bioload is unique to each system based on a variety of factors

The bioload in a reef aquarium increases through both the acquisition of new specimens as well as the growth/reproduction of established livestock. So, unless fish and corals are dying in significant numbers (which they shouldn’t be unless there’s a major problem), the bioload in any reef system is usually trending upward.

While it’s exciting and rewarding to see our tanks bustling with life, we all know there is a certain threshold beyond which a system contains more organisms than it can reasonably sustain in good health. Unfortunately, there’s no alarm on our tanks that sounds when we’re approaching or surpassing that threshold. There are, however, certain signs that tell us it might be time to back off the bioload by rehoming a specimen or thinning coral colonies.

Here are just a few examples:

Stubbornly high nitrate/phosphate levels

We can’t prevent our livestock from producing waste (not even with little corks!), and we can only limit what we feed our fish and invertebrates to a certain extent. So if your nitrate and phosphate levels remain stubbornly high despite doing everything in your power to minimize nutrient import and maximize its export (using RO/DI-purified tap water, employing a quality protein skimmer, performing copious water changes, etc.), there’s a good chance your system’s bioload is simply too high.

Stubbornly low or unstable pH

The more animals you have respiring and producing waste in your tank, the more rapidly buffering compounds in the water will be used up (in other words, the lower the alkalinity, or the water’s ability to neutralize acids) and the harder it will be to maintain an appropriately high and stable pH.

Proper coral spacing has become problematic

Adequate space must be provided around corals and other sessile invertebrates to prevent stinging or other aggressive/defensive interactions, promote good water flow around specimens (so food reaches them, detritus is not allowed to settle on their tissues, and waste is transported away), and simply allow for growth. The more colonies you acquire and the larger they grow, the harder it becomes to maintain suitable spacing.

This can become an issue fairly quickly with fast-growing and/or encrusting species, such as green star polyps, colonial zoanthids, and Xenia spp. (As some of you may recall, I’ve already discussed how GSP overran my former 75-gallon reef.)

As colonies encroach more and more on one another, you may begin to see specimens that once expanded to their full glory every day remaining contracted, physical damage or tissue necrosis at contact points between neighboring specimens, corals being overtopped and by others partially or fully shaded from the light, corals actually being overgrown by others, etc.

Fish can’t get out of each other’s way

A tank that’s too heavily stocked with fish is not only prone to the first two problems discussed above, but also to more bickering and a generally elevated stress level among the piscine populace. The reason being, the real or perceived competition for food and territory is too great and it’s very difficult for individual fish to retreat to their own safe refuge. As a result, all or many of the fish in the tank just seem to be perpetually “on edge” and skittish.

What have I missed?
I’m sure I’ve overlooked some important points here. If you can think of anything further to add, please share it with your fellow salties in the comment section below.


If you enjoyed this post, subscribe to get our new posts in your email.
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. If you have no more money in your bank account, you have enough fish. Take your wife out to dinner and have a nice glass of Merlot, but don’t order fish of it will get you thinking about buying more.

  2. Great post, as usual. One thing I’ve noticed is the growth of algae on the glass. There is a natural cadence to how frequently the glass needs to be cleaned–at times when I have let the bioload get out of control a bit, I’ve noticed that the ‘glass cleaning chore’ is my canary blenny in the underwater coal mine. When the glass gets dirty a lot faster than it should, it might be related to bioload or over-feeding.

Speak Your Mind