wierd sheet algae killing fish

Question
Here's a problem that I've been having for about a year.  It's killed all but three of my fish, and any new fish added to the tank die.

Here's the specs of the tanks I have:
30 gallon community tank
Powerfilter with bio-wheel

50 gallon tank setup for oscars with silver dollars
hot magnum filter
fluval filter

I'm on well water, with a water softener using potassium salt

I started getting some seriously fast growing green algae.  It was different then anything I'd every seen before.  It covered things in layers or sheets.  

The oscar I had at the time was the first to die.  Then slowly every fish in the smaller community tank died.  
I don't see any exterior signs of disease or illness on the fish before or after death.  The only fish that continue to survive are the silver dollars.

I did water changes and used Algae Destroyer.  The Algae destroyer helped a little.  But the algae continued to thickly cover everything.

At this time, my efforts have been concentrated on the 50 gallon tank with the surviving fish.  I've drained the smaller tank.

The latest thing I've tried, about three months ago, was add phosphorus remover to the fluval filter.  This has helped a great deal.  So I thought it might be safe to try adding fish again.

For the first day, things looked good.  But after that the fish (one) started to look sluggish at times and his body would point upward, nose first.  After 48 hours that fish had died.

I did a 25% water change a couple weeks ago.  Even with the water softener, the water is hard.  And the PH is always high.  But years ago, I stopped using buffers because I killed more fish with the buffers than letting things alone.  I misplaced my test kit in an organizational moment over the summer, so don't have any water quality info.

Have you seen anything like this before?

Answer
Hi Andrea
I don't think the "algae" was causing the fish deaths.  First let me get into the algae issue.  From what you're describing, it sounds like it's called smear algae.  It's actually not an algae but a cyanobacteria.  Here's a link that gives a bit more detail on it and let me know if that what it sounds like it is:

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

Now I'm going to jump back to the fish issue.  You don't mention how often you do water changes, but you mentioned doing a 25% water change a couple weeks ago.  All normal stocked tanks should have a minimum of 25% water changes done weekly, along with weekly gravel vacuums.  Uneaten food and fish waste builds up quickly down under the gravel/substrate-and you won't see it.  

If you can't find your test kit, probably should buy a new one.  Ammonia and nitrites should be 0 ppm, and nitrates under 20 ppm.  High nitrates along with phosphates can cause algae blooms.  Nitrates might be present in your tap water-that's common with well water, so I'd test that also.  Nitrates can also build up in the gravel from that uneaten food and waste in there.  And, not doing regular water changes can allow the nitrates to build up.

It's recommended to have a minimum of 55 gallons for one oscar.  And now, the "oscar pros" are even recommending a minimum of 75 gallons for one.  They're a messy fish and grow quite large.  Years ago I had an oscar by himself in a 55 gallon, and I would do 50% water changes weekly to keep the nitrates down, and that was back when it was recommended to keep the nitrates under 40 ppm as opposed to the now recommended 20 ppm.  So, my point is, I'd say water quality was an issue with your fish dying-and also with the algae problem.

Try to stay away from the chemical algae products, most don't work anyway.  Increase your water changes, for now I'd do about 25% every other day, and vacuum the gravel good.  Rotate each side you vacuum, do one side one day the other the next time you clean.  If the phosphorus remover is working, keep using that for now as well.  Cut back your feedings to once a day, what the fish will eat in about 3-5 minutes, and net out any uneaten food.  Do that for a few weeks and see if that helps with the algae any.  If so, then eventually go to a 25% weekly water change/gravel vacuum.  If not, let me know, and let me know on those nitrate tests for your tank and tap water.  If it doesn't get better, and does sound like that cyanobacteria/algae, then you'll need to try the antibiotic erthyromycin, but see if the increased water changes help first.  Most antibiotics will kill off the beneficial bacteria in a tank.  Also, don't leave the light in the tank on longer then 8 hours for a fish only tank, and no more then 10-12 hours for a planted tank.  

Keep me updated and let me know how it goes!

Christy