Curious why algae died off in my tank

I have got a 75 gallon marine tank that's been set up for about 2
months now. All my algae and plant-life has died off in the tank, and
I'm not sure why. I was hoping someone could explain. As it will be
obvious here, I'm not very experienced in the hobby.

I started the tank with a minced shrimp to get the 'cycle' going. I
also used some substrate from an established tank. (Established tank 2
years old, 55 gallon FO tank with Emperor hang-on filter.) All water
in the tank is RO/DI water.

I put a 'refugium' in the bottom. It is a 30 gallon tank that I
intended to eventually have live rock, sand, and macro algae.

I put in a damsel and percula clown fish at first. Then I got a hefty
layer of green algae that started work on the fish waste. So I put in
an orange spotted sand sifting blenny.

He kept getting the water cloudy from sifting the sand, so I put in a
fluval canister filter between the pump from the refugium and the
return to the main tank. (I didn't use the pump in the fluval, just
pushed water through it.) That helped. I also added a couple pieces of
live rock to the system, and a cutting of macroalgae. I had some green
algae in the top tank, lots in the bottom tank.

After a while, I added a small puffer to the tank. His messy eating
habits apparently taxed my poorly conceived filter system. I started
reading ammonia of 0.50 ppm. Nitrites and Nitrates were ok.

After a while, I noticed my algae in the tank all disappeared over the
course of a couple days. It had totally infested my lower tank. Now it
was gone. Only the macroalgae was left, but at least it was growing
ok.

I could see that the live rock had started to thrive though. Lots of
krill, a bristle worm, a fan worm, and other small creatures swarming
the rock and sand.

Next I added a few bioballs in a length of 2' by 4" PVC pipe. The
overflow from the main tank dumped into this pipe filled with
bioballs. I drilled holed in the bottom of the tube so the water could
flow out into the refugium in which the tube was placed. I also added
an airstone at the bottom of the pipe for increase air to the
bioballs.

My ammonia dropped to 0.25 ppm, and still no Nitrates or Nitrites
readings.

I got a new light tube for my bottom tank, which was for promoting
plant growth. It replaced a generic aquarium tube that came with the
fixture. Once I put that in, 80% of my macroalgae died. I don't know
if that was related, or just a delayed effect from what killed the
rest of the algae.

So, why did my algae die? Is this normal? Should I test for something
else?

Paul Vincent Craven

http://www.cravenfamily.com