Help! Rocks and fish fins are turning reddish brown..


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???  Hi!  I am relatively new to tropical fish.  In the past few months I have averted a few disasters by reading message boards like this; however I'm really stumped this time.  I have 2 tanks, a 5-gallon and 10-gallon.  The 5-gallon is not having any problems as far as I can tell, and it has 4 female Dalmatian mollies in it (one fully grown and 3 babies that are about 5 months old).  My 10-gallon tank has 5 male baby mollies (about 5 months old), one guppy, and one Cory cat.  The pH, ammonia, nitrate, and nitrite levels of both tanks are the same (pH=7.0, ammonia=0ppm, nitrate=10ppm, nitrite=0ppm).  I started adding some aquarium salt to the water about a month and a half ago because the adult Mollie (the mother of the babies) kept getting sick with Dropsy.  I read that for mollies a little bit of aquarium salt would relieve some of the stress.  Since she got better and I gave away 7 of my babies the tanks have seemed to do fine (there were 15 babies so my tanks were a little over loaded for a month or so, but I did every other day water changes and I was able to keep the nitrate, nitrite, and ammonia under control). 

About 2 weeks ago I broke the heater in the 10-gallon tank, so the water has been a little colder than normal but the fish seem fine.  I was in the process of completely cleaning out the 10-gallon tank when I broke it because the rocks were really dirty from when I had all those baby fish in there.  Before I realized it was broken I had placed it back into the tank. 

My problem is that shortly after cleaning the tank the white rocks started to get a reddish brown coating on top of them.  I didn't worry about it too much because I thought it was just some sort of algae, but now 2 of my mollies are getting reddish brown stains on the rI'm of their dorsal fins.  They are not acting weird, and they eat like pigs so it doesn't seem to be bothering them.  I was just wondering if anyone else has had this problem, or if it was something I did that is causing this?  I checked the heater to see if it may have had any mercury in it, and I don't think it did.  Could I have introduced some chemical or metal into my tank from the broken heater?  If so I'm not sure how to handle the problem except to clean the tank out again, but I don't even know if that will solve the problem.



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Yikes! What a dilemma! It sounds like you're growing algae on your rocks. I've never heard of fish growing algae on their fins, though. What do you keep the temperature at in your aquariums? While mollies can certainly survive lower temperatures better than some other tropical fish, they still don't like them. The fact that your fish is getting dropsy is not a good sign.

Okay, here's what I'm thinking. It may be temperature. Another thing that comes to mind is tank size. Mollies are active little critters and prefer something bigger than a 5 gallon aquarium. My final thought (sorry, but three thoughts in one day is really pushing it for me) is that perhaps you are overfeeding. I've seen this lead to dropsy and it would be providing a great source of nutrients for brown algae to thrive.

That's all I can think of for the moment. Hope it helps, and I hope we can get this figured out.

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