Mycobacterium and alage problem

MattW · May 8, 2026

A little backstory. My biggest tank has had a dreadful last few months. Over half the original stocking is dead, alage runs rampant, and symptoms of Mycobacterium in the remaining fish are starting to show again after a short hiatus. Medication has no positive effect on the fish. Frequent water changes and a complete reduction in liquid ferts & lighting have little to no impact on alage. Overall, these issues have started to put me off the tank. The worst part. Is that the fish were part of my first breeding project, and seeing them wither away just annoys me more.

So after doing some reading and thinking about the current situation, I think I have found out why it's gotten so bad. According to what I read, algae, along with mycobacteria, thrive in high-nutrient settings. Originally, the tank never had any spikes or this type of issue. However, when gravel vacuuming an area of the tank with only gravel recently, it picked up a large amount of dark black/brown particles, which I'm starting to think was pond soil. The strange thing is, I had pond soil on the bottom with a good-sized cap of gravel above. The only thing I can think of is that either something has dug down? Or the substrate has sunk, and soil has made its way through the cap? Either way, there must be a leak of nutrients going into the water column, leading to the alage and mycobacteria problems.

Is my theory even correct? Or could there be something else causing the issues that I'm missing?

MattW · May 9, 2026

Did an 80% wc today and a gravel vac. Found some soil in the bucket at the end again. Reckon this tank will have to be torn down sooner than planned

GaryE · May 9, 2026

This one sent me off reading. It does appear possible that fish can pick up Mycobacterium marinum from eating detritus, and soil escaping capping in a tank could be functionally that.

Most sources focus on fish scavenging infected tankmates, but detritus is mentioned.

The pathogen probably came from the fish. I have found tb rates higher in harder water species, like your Sawbwa resplendens. The fish usually encyst and control it, but if they are exposed to a rush of the bacteria, it can overwhelm them. Assuming it's in the detritus, that would seem possible here. A fish died and wasn't immediately seen, and its pathogens found a waiting place in the close to the surface soil.

As for the soil 'escaping', do you have burrowing snails? They'll do it. Or a water change hitting the substrate with too much force...

MattW · May 9, 2026

It does seem more likely to be caused by detritus rather than scavenging the infected. But with this tank's issues, nothing can be crossed off I guess. :dunno:

The first instance of it that I recall was my original breeding group. At the time, I had two majorly affected fry out of 20 or so. They were kept behind & didn't last very long. While the others were transferred to the main tank to join the parents. Nothing had popped up for a long while until Dec & Jan this year. Then the symptoms and death came quickly.

Thankfully, I don't. Only ramshorn snails occupy that tank. I addressed that issue with a pond basket to disperse water going into the tank. The original substrate didn't have soil in the front 10-15cm of the tank, so I always pour new water there. Perhaps that area has had soil leech into it as well.