My tanks just crashed, white fuzzy stuff on dying corals

[smaurer]

Not sure why my tank crashed. I have a 15 gallon nano reef that has been running for a little over a year with no major issues, I moved the tank once 6 months ago with no major problems. On saturday I changed the filter and carbon, and did a 50 percent water change which is more than I usually do, usually I do 3 gallons every two weeks. Also cleaned a little more sand than usual. I have been battling nitrates that entire tanks life (10ppm plus). right after the water change all my inverts died and my corals started to grow a white fuzzy material on them similar looking to cotton. Now they are all completely dead. My single clownfish is still alive. I checked all my parameters Tuesday (3 days after water change) and nothing was high other than nitrates which have been at 10ppm. Calcium was a bit low (260ppm). pH 8, ammonia 0, nitrite 0, phosphate 0, I also had a diatom and hair algae bloom right after the water change. temp has been kinda high, reaching 82. What have I been doing wrong? This may not be worth asking because I should have my 40 breeder starting to cycle very soon and all i have left is a single clownfish. But it was really frustrating to watch my corals die virtually overnight. Any ideas? I don't want this to happen to my new tank I am setting up. Thanks for the help in advance. Most of my corals are softies, I like the movement in the tank. Also, this tank does not have a skimmer. and I feed the clown once every 2 days. Ive heard mixed things from feed 2 times a day to feed once a week. I picked somewhere in the middle.

[whodah]

I know bad things can happen when disturbing an aged DSB (Deep Sand Bed), that'd be my best guess. But it doesn't sound like it was that deep either. Still my best guess.

[snakeman]

What is your water source?

[wonrib00]

Once a week is not enough to feed a fish, min every other day.

Did you switch salts? How are you testing the water, new kit? What's the salinity?

[smaurer]

Sorry, I didn't check this after no one had replied for a while. sand bed is about 3 inches deep, water source is kingsburg city water ran through a 4 stage BRS RO/DI filter that I have ran around 40-60 gallons through, for the last year I have used instant ocean reef crystals. I did get a new box of salt about 2 months ago. I am testing the water with API saltwater and reef test kits that I have had since i started the tank a little over a year ago. My salinity stays about 1.025. After I posted this I was thinking that I had sprayed for bugs around my house 3 days before the tank crashed, and my water tube I use for water changes was outside when I sprayed. I thought I avoided spraying close to the equipment but maybe I got too close. Maybe I contaminated the tank and fried everything myself.

[snakeman]

Insecticides can do a number on a tank. There are folks that dip coral in an over the counter insecticide, but they take steps to avoid getting it into the tank. Small tanks also can go through changes rapidly due to lower water volume.

There are plusses and minuses to a deep sand bed. For starting out I would recommend bare bottom or shallow sand and a good skimmer. Red Sea makes a decent hang on the back one.