Slimy walls

[crittermom]

I have a 20 gallon tank in my classroom. I'm just getting it back together since all of my fish died (about 3 weeks ago)when I used a certain water conditioner (back to plain old dechlorinator for me!). I started with just the one female guppy that survived, changed out about half of the water, and used just the dechlorinator. It's been doing great, so I added a couple of high-fin platys last week. Then I got a male guppy and 2 females (one of those died, so now I only have the 5 fish in the tank, plus about 5 baby guppies). I noticed earlier in the week that the tank wasn't clear and sparkling, but looked a little dull...not cloudy, but kind of dark yellowish. This morning, I noticed stuff on the tank walls, and when I put my finger in and slid it along a little bit of the wall, it was slimy. Is this algae? Did I put too many fish too soon? I've been keeping fish at home and at school for over 30 years now and have never had this kind of trouble. I used to keep a bunch of fish in a 20-gallon tank with no problem...are the newer ones less hardy? Anyway, I'd appreciate any ideas on how to deal with this latest problem. (Oh, I had 2 golden mystery snails, both died and now I'm picking babies out of the tank...surely they can't put out THAT much slime, can they?) Thanks in advance! OK...tank is in a corner, away from the windows, but I keep the light on from about 7:30 AM until about 4 PM, if I forget to turn it off, it's on all night. Fish seem to be doing fine.

Nitrate: 40 (a little high)
Nitrite: 0
Hardness: 300 (very hard, don't know how to fix this)
Alkalinity: around 100
pH: about 7

 

[crittermom]

Oops, didn't put the water parameters. I didn't have time to check today, but I will do so tomorrow.

 

[ncjharris]

Sounds like algea.
Is the tank in direct sunlight?

It could also be some kind of bacteria bloom, esp if you have a new tank. Either way, if the fish is ok, then its not much to worry about - other things live in the tank too!

 

[WILDER]

Any signs of fish flicking and rubbing or red inflamed gills, do the gills look shiny, any grey patches on the fish, how often do you maintain the tank, do you clean the glass with an algae scaper.

 

[dwarfgourami]

If the tank is away from the windows, could be brown algae. Harmless but unattractive.