fluffy brown growth and sudden deaths

Question
We have a 75 gallon aquarium with various tropical fish, tetras, guppies, mollies... The tank has been set up for a little over a year. We had an outbreak with the water turning green. I added some green algea chemicals that we got at the local petstore and was careful to follow the directions. Then did a 20% water change. the water started to clear. We had one silver mollie that wasn't looking well. Then one day, very suddenly, the mollie was dead and the tank had fluffy light brown growth everywhere. This growth wasn't visable the day before at all. The dead fish was covered and it spread from there as well as patches everywhere. This brown goop was not stuck to the glass, it was free floating. I used a net to swirl around the water and got nets full of it. That afternoon the fish started dying. Within 24 hours 20 fish were dead, all had this brown fuffly stuff growning on them as soon as they died. I did an extreme water change, 50%. Contiuned to filter the fuff with a net, took out all ornaments to clean and cleaned out the filter. (I had thought maybe it was the green algea dying off and clumping together?) The water looks better. Each time I check the tank, 3-4 times a day, I see new growth of this brown fluff and use the nets to filter out more. The fish look like they are gasping, their gills hard at work. And they are continuing to die. 20 the first day and 4-5 a day since. This tank was well estabilshed and had a great variety of colorful fish. NO tetras have died (neon tetras, glow light, red minor) that just seemed odd.
I've been searching for answers. It's not an algea that grows on the glass, it's brown fluffy clumps tha settle to the bottom but stir up into the water easily. I'm looking to clear this out and hopefully save a few more fish lives.
Thanks

Answer
Jen, it sounds like what you have is an outbreak of Silica Algae. Algae can live in the lowest amount of light, and can sometimes appear to be absent when tested. One main reason why this type of algae grows as it does is from high amounts of nitrite. It sounds like this could be an issue, especially since you say the fish have died. You should monitor the nitrite and phosphate levels to try and rid the aquarium of this algae. Also, low lighting, and the absence of oxygen can play a great role in the growth of this silica algae. As you had mentioned the chemical you added to remove the algae sent your biological filtration outta whack so to speak. The chemical was meant to break down the algae to be caught in the filter. Due to the conditions in the tank water, this allowed the silica to continue to thrive and your filtration system to deplete. The aquarium needs to be cleaned, vacuumed and water needs to be changed, I would recommend everyday, at least 25-50 percent a day until the nitrite levels go down to a safe level and the silica begins to disappear. Also, increase the lighting in the tank believe it or not, this will help it turn to the better commonly seen green algae. This too can also pose a threat to the habitants if kept at high amounts. Also, add a pleco or few to help remove the algae. Make sure the filtration system You have has a proper filter to remove silica. This could be a long process due to how bad you say the problem is. Your tetras are more hardy fish and have probably been able to slowly adapt to the conditions.  The water parameters are probably all out of safe range so I also would recommend purchasing some sort of tests to monitor the quality, I use tests strips that are made by quick dip, gives you 6 tests in 1 strip instantly. Its gonna pretty much be a start over, it will seem. Be sure to monitor the water quality at least twice monthly once it is established and up and running well again to avoid these potential deadly problems in the future. On top of that, be sure to perform regular water changes to aid in the good health of the aquarium, do these at least monthly. Also be sure to not clean the filter too much depending on what type you have, as this will kill the beneficial bacteria that are living in the system that fight off the bad bacteria. If you need any other hints or help, feel free to contact me. Good luck! Hope I have helped.