columbian sharks and weird algae

[fire_fly]

i just bought a powerhead for my 27 gal tank, hooked it up to the undergravel filter and now(2weeks later) theres weird algae growing on everything but the fish. it looks like the same stuff you see growing on the rocks in fast moving streams after the fish are done spawning, and my columbians are dying! no signs of sickness eating extremely well but every second morning i wake up theres anotherone, anyone haveany ideas? ive tried cutting down on the current by placing rocks and more fake plants in the tank but theyre still dropping like flies, any input would be greatly appreciated, thanx
(btw therea a lg goldfish and 2 weather loaches in that tank also, and there were 3 columbians)

[BOAS_N_PYTHONS]

Your situation can be a number of things. You described a few things very well but to give a correct answer back a few other things need to be addressed first to better answer and help your situation.

When was the tank set up?
How long was it before the set up and introduction of fish?
Were all fish introduced at the same time and if now when were all added and times frames between each addition?
Has the tank had enough time to cycle and is there enough bacteria growth to sustain this fish population?
Feeding, what, how often, for how long each session, etc?
Filter, is it clogged, running smooth and is it the right type for the set up and pressure?
Filter does it have enough benefictial bacteria as well?
Substrate is it clean or is there decaying food material?
If all other fish show no signs of illness, the sharks may already be a factor and where they came from?
Chemical balance in the water in PH and overall cleansiness?

This is just a starter of questions, to help better understand your situation to come up with a proper answer or answers for you.

[fire_fly]

the tank was set up about 6 years ago
these are (the sharks) the first new fish i have put in this tank for over a yr.
i feed them once a day in the morning(one really big pinch of tropical flakes) and there is usually no food left over within 20 mins
filter isnt clogged, im doing a 1/3 water change every 2-3 weeks
there isnt any decaying food in the gravel, i give it a quick vaccuume when i do the water change, and the water is crystal clear, no odours
the other fish did look really healthy but the past few days the loaches have been looking a little paler than usual, still eating good tho

do you think it would harm the other fish at all if i started putting in 1 tsp of salt to every 10 gal? because i read that the sharks like a bit of salt

[BOAS_N_PYTHONS]

Salt is ok, people usually buy aquarium salt from pet stores, but quantity to price your better off going to the grocery store and picking up pickling salt with no additives, you get 10x more salt and its cheaper than the stuff at pet stores. You' ll need to disolve it in the clean water that is used in the water changes, before putting the water in the tank. Salt at the ratio of 1 tsp / gal is correct. And its not that sharks like it but all fish do, keeps bacteria down and heals their slimy layer from fights, rubbing up against decorations in the tank and so on.

The algae problem, might be controled better due to the salt a bit, maybe add a pleco or 2 to eat up the algae. Also is the tank near a window, meaning direct sun light against the tank, this can create a algae bloom that goes out of control if the chemical balance is to rich.

[fire_fly]

nope, its not near a window, but my weather loach is acting all weird the past few days to, spazzing out and flipping around the tank, and for the rest of the time hes just listing at the bottom, gasping for air, he also dosent seem to be eating