Coral in freshwater tanks?

Question
Hi Christy~
We have a 20 gall glass tank that we've been setting up for fish for about 6 weeks now (we've never changed water, just thought the filter and time would rid the watner of any problems). I put a piece of real coral that I bought in Florida in the tank early on..it started to green up within a week, but I thought that was natural. Then someone told me you should never use coral in a freshwater tank (uh-oh)!. The tank has been beautifully clear since the first cloudiness went away but before we went to get our first fish 3 days ago (3 neon tetras and an Upside Down Catfish), we noticed brown algae (starts like a thumbprint, now growing like crazy brown calcified slime on inside of glass).  Could this have been caused by the coral? We lost one Neon (he didn't look well to me as soon as we put him into the tank and the other Neons "picked on him"), but so far the other 2 Neons and Catfish are ok. We did a water change (about 10% of the tank water) this morning and everybody's livened up. I'm concerned about the brown deposits on the sides of the tank and wonder if it will hurt the fish (or could maybe the coral have caused it)? Lesson learned:  read up on aquariums before starting out willy nilly!
Appreciate any help!
Linda

Answer
Hi Linda
The only real problem I could see with using the coral, is it may raise the ph in your tank, which may or may not be an issue.  To check it, test the ph reading in your tank.  Then, get a glass of water, test the ph in that.  Let it sit out for about 12 hours, and retest the glass of water's ph.  The glass of water test should be what's going on in your tank.  However, if it's significantly higher in your tank then the glass, then you know it's due to the coral.  Now, most fish we buy are tank bred or farm raised as opposed to being wild caught.  So, they can tolerate ph from 6.0-8.0 just fine=meaning no real need to alter it.  Unless your planning on keeping discus or other sensitive fish, should be fine.  

The brown algae, is called diatoms.  Generally harmless, but it's also common in newly set up tanks.  Here's a link that explains it a bit more, along with remedies:

http://freshaquarium.about.com/cs/maintenance1/p/algaebrown.htm

At this point, I don't recommend adding fish to take care of it.  What I would do, make sure you're doing frequent/weekly water changes of 25% along with weekly gravel vacuums.  Make sure there's adequate filtration on the tank, and make sure the water surface is being agitated by the filter waterfall/output.  The surface agitation is what allows the oxygen and co2 to exchange.  Make sure the light's on for the appropriate amount of time-fish only with no live plants at least 8 hours, with live plants at least 12 hours.  This will help to make the green algae grow which competes with the brown algae for nutrients.  Algae isn't a bad thing in a tank, it's actually a good sign, and the fish don 't mind it at all.  It's usually the tank owner's that despise it :)

Christy