Fish Died So Fast!?

[Tdog]

Hi everyone, i got my first fish 3 months ago, it was a betta. I kept him in a 10 gallon tank with 4 cory catfish. Had a heater set to 79, and a filter. Things were great until a couple days ago. The fish started seeming lazy and my betta got a little tiny light grey spot newr his gills.

I looked in the tank that night and two catfish were just dead all of a sudden.. i scooped the remaining fish out and seperated them.

I started cleaning the tank out. I threw away all the gravel and bleached the tank and heater.

By the morning all the rest of the fish were dead how did they die so fast!? Im really sad because i thought everything was great.

 

[BettaBettas]

this is a hard problem to solve, seeing the lack of information. not your fault though, just wasn't enough to observe.
do you know if they were bloated? any visual symptoms other than grey spot? if it was a grey blotch/spot then it could have been a minor fungal infection on your betta but that wouldn't have killed all your fish. maybe ich?
whats your water parameters if you know?
An when did you get this tank, how big is it to?
 

 

[LyraGuppi]

I'm sorry about your fish, hopefully we can figure out what happened. :c
 
Did you cycle your tank before putting fish in? 
As BettaBettas said, do you know the tank's ammonia, nitrite, and nitrate levels? This will be very helpful when discerning what happened.
The other 2 cories and betta likely died from bleach poisoning. Bleach is really hard to handle when cleaning aquariums, since it can be so lethal. Vinegar and hot water are much better, not to mention easier to work with.

 

[nic1]

Why would you bleach everything in your tank? bleach is very potent and if not used properly even the smallest trace can kill.
Why did you throw away the gravel?
What is your maintenance regime? How much water do you change each week?  how often and how do you clean the filter? what are your water parameters?
what kind of corys do you have?