I think I made my tank toxic. Why?

I don't know if it's enough to kill the fish but 2 capfuls sounds like an awful lot for 33gal. Yes, Prime does smell of sulfur. But again, I don't know if that's enough Prime to cause the deaths.

Prime is safe to dose up to 5X, but may be what accounts for the smell.

I agree with JM that the pH drop was what probably killed.
 
depends on what caused the pH to drop.

how is the kH in the tank? if you have little to no kH you then could run the risk of a dangerous drop in pH .
 
Aren't neons the wrong type of fish to use to cycle a tank?

I thought you needed hardier fish like danios or red/serpea tetras?

I thought neons were very susptible to ammonia or nitrite.
 
I also did not test for kh. I guess it was the drop in ph but still can't understand why.
As for the Prime I don't think it was overdosed. I generally use 1/2 cap in 5 gallons when doing a water change so I dosed for 20 gallons only.

There have been no fish since yesterday afternoon and I kept a sponge filter and a sponge over the intake, so are those 2 sponges viable for bacteria still?
 
I agree the Prime probably didn't kill them. But just thought that I'd point out the Prime I have says to use 1 capful for 50 gals. Maybe yours has a different size cap or a different concentration.
 
AquariaCentral.com