nitrate woes

I posted this question before, and didn't really get any responses that
helped, so I thougt I'd post again with an update. I have a 55 gallon with
10 or 15 small fish. I have a large group of red temple plants and a large
group of Eusterallis stellata and a few other misc plants. I was adding 20
ppm nitrates every day and when I would test I would always get 0. (test
kit seemed ok) Fish started to get sick. I discontinued fertilization for
a week to let the fish heal, and the stellata started to turn to mush and
break apart. I started fertilizing again at 5ppm per day and the fish
started to get sick again. I've since moved the fish into another tank so I
can dose the 55 at will, but I would like to put the fish back eventually.
I also have crazy brown and beard algae all over the place which I've never
been able to get under control. Here are the full specs of the tank

55 Gallon standard tank
4-32 W 48" flourescent tubes (4100K)
Substrate of laterite under inert gravel
flourish tabs
diy C02 (3 2 liter bottles, replace the oldest every 5 days) according to
the chart, I'm usually in the mid-20's
Using PMDD ingredients to fertilize, though not at PMDD rates.

What is my problem? Should I give up on the Eusterallis? I could live
without it, but the algae is driving me insane!! You guys with all the
pictures on the internet make it look so easy.... :-)
Thanks! Allison