Salt water tank

Question
Hi,  I was given a friends used set up for a salt water aquarium. 50gallon tank, protein skimmer, turbo pumps etc as well as at least 44 pounds of live rock. I have taken several months to get it all ready (cycling rock etc).  I added an undergravel filter which is run with 2 airstones and an airpump.  A week and a half ago I finally added 6 fish (3 yellow tailed damsel and 3 blue damsel fish).  Within 24 hours 2 of the yellow tailed damsel fish died.  The remaining 4 fish are all alive and well for the last week and a bit.  My question relates to the water - there seems to be a rust coloured sediment on top of all the live rocks and pebbles.  I siphoned off the gravel on Friday and it has all recollected.  Do I need to put the turbo pump back in the tank to increase water movement? (I took it out because it made quite a few bubbles).  Is this just the adjustment phase of the rocks.. finally getting to do some work with the fish wastes?  What is going on?  Any help you can give, I would appreciate.  I would like to buy more fish but will wait until I have this rust colour fixed.  The fish are eating fine and look great.  I tested the water on the weekend and it is all ok (nitrates at 25mg/l) but this is still ok levels.  What you suggest?

Answer
Bronwyn,                                                               I don't know what etc is, but I hope it ends with I have a nice carbon canister filter. Without that you can not remove the toxic wastes from your tank. I personally don't use live rock unless I am running a reef system. Get rid of the air pump and add power heads unless that is your turbo pumps you are talking about. Movement of the water just envelopes oxygen into the water. It is not a form of filtration. If you cycled your tank out with live rock there should be plenty of bacteria to have sustained your fish.Especially damsels they are very hardy. The brown algae are diatoms .... turn off the light they will grow slower. I am thinking you don't have enough filtration on your system. Let me know what else you are using and we will go from there..You need to run power heads on your under gravel..this is your biological filtration... It brings your water into your bacteria bed faster and your carbon is your mechanical filtration.. It removes your solid wastes and absorbs your extra wastes.... Tina