Strange animals in my tank and corals looking bad

[kitsune]

I recently purchased an established 20 high saltwater tank. Everything was going good until I noticed some strange creatures emerging. First there there was one, then all of the sudden they're all over the place. They seem to be nocturnal as they only come out en mass at night. They look like slugs with a poor attempt at a shell on their back (it seemes to be there, but at the same time, flat) They have a round sucker mouth and two (antenna?) that are set back a distance from their mouth. They are tan in color with their shell being a little darker. I do believe that I observed one eating a small tube worm. Being as it is a reef tank, one could surmise that I was not happy about it! Also, two growths of coral are mysteriously missing. I don't know what is going on here, or if they are dangerous.
My second problem is that my Coral Beauty died and my Pipe Organ coral isn't looking so hot either. My tongue coral isn't coming out all the way. The only things that I added was a coral banded shrimp (who by the way is doing just fine) and a flame scallop who looks a little worse for wear. I have also noticed some filament-like algae growing suddenly, and I don't believe that it belongs there.
I suspect that it may be nitrates. I know that I fed too much. But other than that, I'm at a loss. Please help!!!!!!
Sorry for the long message!!!!
Sara

 

[sbustobante]

hmmm, sounds like the strange creatures you are describing could be limpets. Limpets look a bit like snails, only with a flatter shell resembling a chinese hat. Sometimes the margin of the shell will be covered with tissue. If this is it, limpets are harmless algae eaters.
Now, the rest of your message would seem to indicate you haven't actually performed a test for nitrates. If not, you might want to take a water sample into your lfs to see if you have any problems - it sounds like you might.

 

[kitsune]

Thanks alot! I appreciate it. It makes me feel a whole lot better to know that these guys may not be so bad. Yes, I'll get that tested. I was given a tester, however, the chart that interpreted the results is missing, so I will need to take it somwhere else. Again, thanks!
Sara