Need help with brown algae

Hello,

Here the setup:

50 gallons sea water (salt) aquarium
Start last November using sea water as base and addition of marine salt to
reach proper level
Use beach sand and rock for decoration
Back to a windows, but window aquarium side cover to prevent light from
getting in
Only room lightning at first
Temperature at 25c
Every water parameters are inside norme
First 3 fishs as been introduce (Blue Dansel), they are still OK
Next 3 snails (1 is now dead) and 2 clown fish, both die

Here the problems:

As been OK until mid march, next appear brown algae, mostly in the morning,
so we add artificial lightning using one Metal Halide 175watt lamp. Brown
algae still there in the morning but clean a bit during day (when light is
on), but never dissapear.

Later appear green fungus, look like algae, but are probably bacteria (I
don't know the name). Everything become cover with that green algae and its
rise quickly since we add lightning.

We try to cover the aquarium for 4 days (no light at all) to get rid of the
green bacteria, most of it die, but we still have a greenish water and they
have begin to reappear the next day we remove the cover and start artificial
lightning.

We keep light on for 8 hours each day.

Those algae or bacteria give a bad smell to the water, but the fishs do not
appear to care.

I did try to change a 1/3 of the water, this did not change thing much. The
next day, the water was as bad as before I change water.

Any help to solve the problem will be welcome.

Bye
Jacques