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Black Mold or Algae?

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I have a reoccurring issue with a black moldy looking crud that continues to show up on my white sand. At first, back in April, I blamed it on some API root tabs that I used on some Vallisneria that I had just planted. Now, I’m not sure if it was the cause or not. 

The picture doesn’t really show it as vividly as live. I starts out looking like maybe some charcoal dust got sprinkled around. After a week or so, it will form patches that when vacuumed up look like a loose film that only breaks apart in pieces. 

I started this tank back in March and because of COVID didn’t get fish until August. Before adding the fish I did treat the tank with Chemiclean with no positive results. Vacuuming it seems to be the only cure (haven’t tried anything else). It always comes back after a few weeks. 

This is a fully cycled 60 Gal tank with more than enough filter for the 7 small Brichardi and 6 mystery snails that inhabit it. All perimeters are 0 except Nitrates which stay around 20ppm.   



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Thanks for the quick replies. 😎
I’ve got new babies and another pair that look like they’re planning a family. Don’t want to do anything to upset that. Is erythromycin safe in this situation? Would it hurt to try the Chemiclean once more since I already have it?

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You say it goes away when you gravel vac but comes back a few weeks later. Are you doing any water changes after the initial gravel vac or are you waiting the few weeks to do the water change with the gravel vac? If you’re waiting, that could be your tanks way of telling you that you need to do a more frequent, a more larger, or more frequent larger water changes in between gravel vacs.

It also looks like it could be a dissolved root tab that has settled towards the surface of your substrate. That powdery charcoal description sounds a lot like a dissolved root tab. If that’s the case, it is possible that Cyanobacteria is growing on top of the nutrient dense layer exposed to the water column. When I use root tabs, I make sure to put them touching the bottom glass, all the way under the substrate. If it is a root tab, you could be disturbing it by doing the gravel vacs and therefor shifting the dissolved root tab around enough to come through the substrates surface.

You also say it’s an established 60g. How deep is your sand bed? Is it possible it’s gone anaerobic and that’s the black that you see? If it’s an anaerobic layer of substrate, you could also be disturbing it by doing your gravel vacs. 

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I do water changes at least weekly if not more often. I’ve been under the impression that I have high nitrates but now I’m not so sure. The one test I was using showed solid orange 40 ppm but two other brands of tests tell me that I’m actually closer to 10 or 15 ppm. Who to believe?🤷‍♂️

I think the Cyanobacteria bacteria diagnosis is correct. I did a little more looking around the tank and found a couple of dark red patches on the back glass. I have seen the red slime in this tank before on a rock. At that time I gave all the rocks a good scrub in a hydrogen peroxide bath.  I’ve also scrubbed off black crud off of a driftwood ornament. 

I’m guessing that this came to the tank from my daughter. This was her tank...a gift that I gave her 4 or 5 Christmas’s ago. I got it back when she moved out in the country and started raising fancy chickens...in addition to two rowdy boys and demanding job. She had it as a heavily planted tank. When I got it she had sold her fish and plants but the tank was pretty neglected. I got it cleaned throughly but I suspect that something entered on a decoration...probably the driftwood. BTW, the driftwood was a store bought piece and not something found. 

I checked out the ingredients of Chemiclean and it is erythromycin, just a different form than the API brand. I have no idea what the difference is.

I’m going do a good water change today, remove as much of the black stuff as possible and add the Chemiclean. 🤞

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