I would buy a liquid test kit (the dip tabs aren't all that accurate). Find out where you stand with ammonia, nitrItes, nitrAtes.
Find out what the other fish is or post a pic.
Is the "black" algae brown, red or green?
I am assuming the sponge is in your HOB
(hang on back) filter. If so, you should rinse it out when you do your PWCs (partial water changes) in the old SW
that you siphon out of your tank. This will keep your beneficial bacteria alive.
You should do PWC
about every two weeks or maybe even three. Pre mix your SW
for at least 24 hours. Have a ph and heater in the mix and try to get the salinity and temp the same as your tank's.