The concept of a limiting factor isn’t a common subject in the aquarium hobby, but it should be. In any tank, there is a list of things that can and will hold back the fish or the tank as a whole. It is important to understand this concept because all of your efforts on other factors can be completely pointless, and sometimes even harmful, if you don’t understand and address your limiting factor. In most cases, there is only one limiting factor, but certain things can work together in many tanks to be simultaneous limiting factors.
At a very basic level, in a freshwater fish tank, there are certain things that can hold back the fish and their ability to thrive. Water changes and water quality, food quality, tank size, filtration, pH, etc. can all hold your fish back. If you do 25% water changes every few weeks and feed okay food, both are limiting factors. If you increase your water changes to every week and step up to larger and larger, the water changes and water quality will no longer be a limiting factor, at that point it is just the food.
African rift lake cichlids are native to very hard, alkaline water. Some people will tell you to use cichlid lake salts and buffers, others will tell you they make no difference. The difference they can make are dependent on two main things: 1 – the water’s natural chemistry and 2 – the limiting factor of the tank. If the water quality is bad, the perfect pH and the right salts won’t produce significant results. However, if the water quality and food are both very good, and the natural chemistry of the tank is not hard, adding the salts and buffer can produce a noticeable improvement in the fish. They can have better colors, better growth, more natural behaviors, and fewer health problems when all of these factors are ideal, not just one or two.
Planted tanks help illustrate this concept even more. There are more factors at play in this situation. You not only have water changes and water quality, food quality, and the other factors of a fish only aquarium, but you also have lighting, nutrients (nitrate, phosphate, etc.), iron, carbon/CO2, trace elements, and even photoperiod. If your lighting is minimal, there is no point in adding CO2, iron, or trace elements. These can all be perfect and they won’t make one bit of difference because the lighting is the limiting factor. In fact, messing with other factors before you have addressed the lighting is almost certain to produce algae problems in the tank. Until you upgrade your lighting, nothing else will help. This is why it is important to understand the concept of a limiting factor in general as well as what the limiting factor is in your tank.
Planted tanks also illustrate the idea that there can be multiple limiting factors. The factors in a planted tank need to be in balance. You may upgrade your lightings, but you may now have multiple limiting factors. Perhaps your nitrate and phosphate are no longer adequate and you need to start dosing them. Once you correct those limiting factors, perhaps now the carbon source and iron are holding back the plants. Once you correct that, perhaps now it is the potassium and trace elements that are the limiting factors. You can see how there is always something holding back the tank. Sometimes it is one, sometimes it is multiple things together. But focusing on potassium or carbon is pointless if your lighting is still the limiting factor.
Reef tanks are probably the best example of the need to focus on one limiting factor at a time. There is no point in dosing calcium, alkalinity, or magnesium if you don’t have your nutrients under control or adequate lighting (or if you don’t have stony corals). Usually, the first limiting factor is lighting. It is very common for people to have minimal/inadequate lighting in a reef tank. Good lighting isn’t cheap, and this discourages many people from getting the right lights from the beginning. The other most common limiting factor is nutrient control. Keeping nitrate and phosphate under control can be very challenging for new reefers. Until you take care of these limiting factors, there is no need to think about calcium or anything else more advanced yet.
Reefs also provide a perfect example of having multiple limiting factors. When calcium, alkalinity, and magnesium become your limiting factors, you do need to address all three simultaneously. You usually hear about calcium and alkalinity, but magnesium actually inhibits the other two from precipitating with each other directly in the water column. So until your magnesium is above 1300, don’t even worry about the other two because you may end up dosing like crazy because the magnesium isn’t there to stop them. Once you correct the magnesium, then you can address the calcium and alkalinity together.
Once you get that trio figured out, you may need to start addressing the other elements such as potassium, iron, iodine, etc. Red Sea has a great line of products as part of the Reef Care Program. These are tests for specific elements and additives you dose as needed based on those specific tests. There is no point in even thinking about this if your lighting sucks, your nutrients are out of control, or you don’t have your calcium and alkalinity straightened out yet. Until all those other things are corrected and optimized, the elements in the Reef Care Program aren’t your limiting factors and won’t make a difference.
This is Why Some Products Work So Well for Some People, and Not at All for Others
There are many products that you will find get very mixed reviews (imagine that, conflicting opinions in the aquarium hobby!). For some people, certain plant additives make all the difference, other people see no difference at all. I think many of these cases are so because some people use them when they are not the limiting factor. If you start using iron before your planted tank even has enough light, you aren’t going to see a difference. If you use AcroPower on a reef before you get the calcium and alkalinity corrected, you may see minimal or no improvement in coral colors.
The use of carbon has been shown to have harmful effects in some planted and reef tanks. In those cases, whatever good thing(s) it is removing is a limiting factor. In other systems with other limiting factors, the use of carbon on a planted tank may produce no difference, or even help improve things. These drastically different results with the exact same products clearly demonstrate the power of limiting factors. Only very well-balanced planted and reef tanks will have visibly negative effects by the use of carbon. Understanding this concept can help you focus your efforts on the factors most likely to produce the greatest improvements in your aquarium. (Note: Even well balanced tanks may benefit from LIMITED use of carbon, such as for 24 hours every month. This removes discoloration and some negative things without constantly removing the good stuff.)
There is a long list of products that fall into this category. The reefing side of the hobby is overflowing with products that may or may not make a difference for a particular tank at a particular point in time, but freshwater has its share too. Everything from slime coat promoters, waste reducers, bacterial additives, trace elements, certain foods, plant additives, goldfish conditioners, softwater additives, cichlid buffers, filter aids, and more.
Try Stopping the Use of Certain Products Sometimes
There are certain things we know for sure are beneficial, such as water changes, good food, calcium in a reef tank with stony corals, etc. But there are also many things that don’t always make a difference, we aren’t sure if they are the limiting factor. In these cases, it is best to try running the tank with and without it. For example, some things we just can’t test for (amino acids for corals, every trace element for plants, etc.). If you are dosing one of these types of products, let yourself run out. Try running the tank with it, then when you run out, don’t replace it yet. Sometimes we may have had a gradual benefit that we didn’t notice because it was so slow to change. Other times we thought we saw an improvement when we started it, but in reality it was coincidental with some other change or completely the placebo effect. Either way, running the tank without that product for a while can help verify its benefits (or lack thereof).
I remember at the store we used a whole bunch of different Brightwell Aquatics products on our coral tanks. We were using zooplanktons, phytoplanktons, amino acids, and more. Every day we were going around to the coral tanks and adding 5mL of this here, 20mL of that there, etc. Eventually, we got sick of using them all and cut them all off completely. There was no difference. We didn’t see any corals change in any way. Nothing lost its color, shriveled away, etc. This isn’t to say that Brightwell’s products aren’t any good, they just weren’t the limiting factors in our tanks at that time. And they are far from the only company with an overwhelming laundry list of products that you as the hobbyist have to sort through and decide which, if any, may help your tank in any way possible. Trying a product, then stopping its use, can help you decide if it makes a difference.
Focus on the big factors first. Once you make any necessary corrections, then you can focus your attention on the next factor (while still maintaining the previously addressed factors). By recognizing and focusing on the tank’s limiting factor, your efforts will be much more likely to produce the desired improvements in the fish, plants, coral, or tank as a whole.