Cotton-like stuff spreading in tank (but not in fish)

  • #1

I've searched this forum for what is going on with my tank, to no avail (or I may have missed a post).
Since early September, a cotton-like stuff has grown in my goldfish tank, but not in any of my fishes - they still have disposition and appetite. It's just that looking at that white stuff is disgusting and confusing to me.

In the last 3 weeks, I've been vacuuming the stuff from gravel and some walls, and replacing 20% of the water every other day. I was going to use API Pimafix, but when I used it for a fungus infection years ago, fishes got agitated, lost appetite and a weird foam formed at the top of the water. So this time I used Jungle Fungus Clear - it's been 3 days and the white stuff is still there, laughing maniacally at me :)

If this matters (and it may just be a coincidence), this started around the same time I introduced Bug Bites from Fluval to my fishes - I stopped using that food 2 weeks ago.

I would appreciate your help to identify what this cotton stuff is, and how to remove it for good.



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  • #2
I would guess some sort of algae growth. For starters, I would clean up as much and as you can with a 50% water change, black out the entire tank with a towel or blanket for about 3 days, and do not feed the fish during the black out. Using meds to treat anything other than fish disease is a pointless endeavor IMO and may stress out the fish more.

As for possible reasons for the bloom, we need more details. How many and what kind of fish? What size tanks? How often and how much are you feeding them? Can you give number results for ammonia, nitrite, and nitrate? How long do you leave the light on? What kind of filter does the tank have?

  • Thread Starter
  • #3
Hello Demeter,

Sorry for the delay getting back to this. As requested:
How many and what kind of fish?
3 goldfish (2 golden and 1 black moor), all 3-year old

What size tanks?
45 gallon

How often and how much are you feeding them?
Early in the morning and 6pm - I give them 2x veggie pellets (OmegaOne) and 2 pinches of OmegaOne flakes

Can you give number results for ammonia, nitrite, and nitrate?
Ph: 6.8, Ammonia: 0.50, Nitrite: 0.50, Nitrate: 5.0

How long do you leave the light on?
4 to 6 hours daily

What kind of filter does the tank have?
2x Seachem Tidal 55

I also have a 20 gallon tank I had to buy to put another goldfish which was always agitated and bumping into the other fishes. Interesting thing is, the water there has no cotton-like stuff and it is always transparent - I don't give the veggie pellets because that goldfish won't eat them, but she is doing fine with flakes.

I may be taking a vial with the cotton-like stuff to my nearest Petco to see what this might be. I've done 50% water changes every 2 days, but if I leave 3 days without water change, the cotton stuff comes back.

  • #4
Looks like a fungus to me. The usual cause for tank fungus is an abundance of uneaten food particles in the environment.
The fungus itself isn't harmful.
  • Thread Starter
  • #5
Thanks A201 - I tried API Pimafix and Jungle TB630W Tank Buddies, with no results.

My next move is to take the fish out into a large bucket with water and salt, completely wash the tank and filters, use 50% of the water from the smaller tank as a starter for the larger tank, and put the fishes back.

Not a big fan of chemicals, but this (thing) has been challenging to get rid of.
  • #6
A fungus outbreak is often associated with newer set ups. Not enough beneficial bacteria established in the substrate to consume the excess nutrients.
The ugly fungus situation is temporary as long as over feeding isn't an issue & a regular weekly water change schedule is maintained.
No need to break down the tank. A lot of work involved there. Lol
  • #7
Can you give number results for ammonia, nitrite, and nitrate?
Ph: 6.8, Ammonia: 0.50, Nitrite: 0.50, Nitrate: 5.0

This right here raises some red flags for me. There should be no ammonia or nitrites on a cycled tank. Having these means your tank is still cycling and that could have something to do with the fuzzy goop too. I switch to feeding every other day for a week or two, keep up on the water changes too. You want to reduce the amount of ammonia being made by the fish, less poo means less ammonia. Once that is under control I feel they will be fine with one feeding a day, or two smaller feedings. Who knows, perhaps the fuzz is uneaten food, this will fix that too! Also, it would be a good idea to use Prime, it will neutralize the ammonia and nitrite so the fish won’t be effected by it.

I am curious to know what caused the cycle to get messed up. Did you recently change the media in your filters? That would remove most of the good bacteria. Perhaps the meds you tried before have killed off the beneficial bacteria resulting in high levels of ammonia and nitrite.

You aquarium’s little ecosystem simply has to find balance, once it does the fuzz and levels will settle down.
  • Thread Starter
  • #8
Just wanted to update this thread... I used CooperSafe and it's been a little over 1 week and the white stuff is gone - it worked!

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