In this article, we will talk about growth-inhibiting toxins and why you should care about them. People call me constantly, “Gabe, my fish aren’t growing. I don’t understand why. My other fish did just fine.” In essence, we’ve all run the experiment in one way or another. Growth-inhibiting toxins are hormones that larger fish release. They release these hormones in nature so that they discourage the possibility of rivalry.
In other words, the big discus are always the ones that control the breeding population. They can’t get displaced because if other fish swim in too close proximity to them, they get bombarded with these hormones that restrict the growth of the smaller fish. These hormones are nature’s way of selecting the largest, healthiest fish to stay as the breeding individuals and the dominant genetic material.

How Growth Inhibiting Toxins affect Discus Owners

People usually start a tank, and later on, they want to add more discus to their tank. They order the smaller ones because they are less expensive. By then, their original fish have already doubled or tripled in size, and then I always get the same callback where they tell me, “Gabe, what’s going on with the new discus? They’re not growing at all.” The larger discus are doing great, and the problem seems unclear until it comes out that they have put the smaller fish in the same tank or the same system as the larger ones. This would explain they’re not growing as quickly. The problem is the growth-inhibiting toxins or hormones. This phenomenon of growth-inhibiting hormones is being studied by universities in different species of fish and crustaceans and is the subject of several academic papers.

My Experiential Learning Process

When I first set up the hatchery in 1997, I put it on a central system filled with 125-gallon tanks down the center. I remember that I had larger discus in the system, and I would put in the smaller discus, and the smaller discus would not grow, would not thrive. A lot of the time, they ended up stunting. I was confused and discouraged until I realized what it was.
Nowadays, we have all the tanks independent. The only central system that we keep in the hatchery is the breeding section. The reason for this is that we realized that when one pair triggers, the mating pheromones are going to trigger the rest of them. Everything else is independent.

Misinterpretation of Anecdotal Results

Essentially, this article is to suggest that it’s not a wise idea to mix different sizes. Mind you, after 25 years, I’ve heard it all. I’ve had people call me back and say, “Hey, Gabe, that discus you sold me was two and a half inches, and the rest of the fish in the tank were five, and now he’s almost their size. I don’t think your theory holds any water.” That’s like the saying; grandma smoked till she was 100. The way I look at it is, “How long would grandma have lasted if she hadn’t smoked?” Do you see what I’m saying? The fact that the new discus grew that much means you wasted his potential to grow to eight, nine, or even ten inches. He had the genetic potential to grow a lot more. Being in the same water as the larger fish stunted his growth, even though he grew to the size of those in the tank. I would suggest that if you’re going to buy discus, try to get them all the same size. You want the differences in sizes to be no more than 1.5 to 2 inches. You don’t want to try to mix fish with a size discrepancy greater than that. Otherwise, you’re going to create a situation where the discus will secrete the growth-inhibiting toxin or hormone, and what you’re going to end up with on the smaller discus is owls. The reason why I say owls is because the eyes continue to grow. What ends up happening is if the body is not growing and the eye continues to grow, the eyes will be disproportionately large for the size of the fish’s body.

A Suggestion To Offset The Cost Of Discus

Because of the higher cost of individual discus, the hobbyist discus keeper needs to know this topic. When you’re investing $30, $40, $50 on a fish, it’s a wise idea to consider the possibility of growth-inhibiting hormones first. Most people don’t even know about this. I get calls all the time, “I bought discus from another breeder, and they’re all stunted.” The truth is, the blame may not lie with the other breeder. Because they bought smaller discus than those they already had, the growth-inhibiting hormone kept their new fish from reaching their full potential. I understand everybody wants to save a little bit of money, and that’s fine. I always try to ask, “what size are the discus that you have right now?” Try to match them and explain to people why or why not they should do it this way.
If you still want to try to save some money by buying smaller discus, my suggestion is to put them in a smaller tank away from the larger discus, and then that will give those smaller discus a chance to grow. Remember, it only takes about a year to hit five inches if you give them a good diet and good water changes. If you want to do it that way, by all means, but please keep in mind that they will get stunted if they’re in the same tank or the same system as the larger discus because these hormones go straight through the water.
If you have ever tried to put different-sized discus together in the same tank or the same system, you have seen this experiment play out the very way I’m describing. When I was a beginner hobbyist, I did the same thing. I introduced smaller discus and realized that they weren’t growing, and I couldn’t figure out why, but there was nobody to tell me why. So I’m here to tell you why. If you have a central system with multiple tanks, it’s not wise to use that water for different-sized discus.

Separating differently developing sibling sets

Now, the skeptics are going to say, “If what you’re saying is true, then what about the babies when they’re born on the sides of the parents?”
The babies are not affected by growth-inhibiting hormone because they are on the parents’ sides, eating their body slime. In doing so, within the proteins they pick up, they pick up something that neutralizes that growth-inhibiting toxin and allows them to grow. Once they’ve stopped grazing off the sides of their parents, if you keep them in there too long, the parents will stunt the babies. So please take that into consideration.
When we have a big spawn of 300 or so brothers and sisters, after a month, after the second month, after the third month, we have to separate that spawn because some will mature faster than others, and even among siblings, they’ll stunt themselves. So we try to take out the biggest ones and separate them to another tank, to give the second batch, the original batch, a chance to grow, and so on until we have maybe three or four tanks from one single spawn.
You have to do it that way because if not, there will be a group that mature faster than their siblings, and then the next thing you know, they’re inhibiting the growth of their brothers and sisters.

Conclusion

Growth-inhibiting toxins do exist. We’ve all done experiments if you’ve had discus for a while in your own tanks. Word of advice is to try not to mix different sizes. When you do buy your discus, try to get them all the same size. It’ll help you. It’ll help your discus grow together, less bullying against the smaller ones as well. As usual, thank you for your interest. We apologize for taking so long to produce more content. It’s been a crazy year with this COVID hibernation and everybody becoming a hobbyist. I’m trying to find more time. We have a couple of surprises for you this year. We are expanding. We just signed a lease on the new property, and we will be bringing you lots of content on how we set up that gigantic place. It will be as Jack and I talked about when I set up this original place, how the hell are we going to fill this space up? Nowadays, I can’t even fit a 10-gallon in the original hatchery. We’ll see how the new hatchery works out.
We decided that we were going to keep both because this place has been around for 25 years. We look forward to creating more content with you and for you. I do apologize because we have been very swamped, and I didn’t have time. So welcome back; I’m here for you.