Question about diseased aquarium I bleached

So basically around 8 months ago, I purchased a new aquarium, not a very large one 27 gallons so I will refer to that one as the small one and set it up as a freshwater aquarium, unfortunately within 2 weeks of adding the fish they all contracted some type of disease and died within a few days, the aquarium had sand and live plants with a few decorations, I did try and save them with medication but it didnt work, anyway since then I purchased a larger aquarium and set it up differently, but I wanted to clean the smaller aquarium and get rid of all diseases to use it as a quarantine tank, there was a little bit of water left in that aquarium with the medication in and the ornaments I assumed the diseases would die off until it came to emptying the aquarium, despite there being no or very little fish waste in it when I moved the sand a disgusting smell came, there was white residue like streaks across the surface of the water, patches of some black growth with white around it and the bottom layer of sand was greyish black and smelt like a sewer.

So im not sure exactly what has happened that it went so rotten, anyway I removed all the ornaments, sand and airstones and filter, cleaned the aquarium out, scrubbed the gross stuff off and then filled it up with bleach water for 20 minutes and sprayed bleach on the outside, I then realised my bleach solution may not have been strong enough and so I emptied the tank and sprayed pure bleach on the inside and outside with a spray bottle, but I forgot to on the bottom, I also sprayed bleach on the lid and threw the lighting out, I then left it like that for 40 minutes, I then rinsed the aquarium with hot water because I forgot to bleach the outside bottom part.

Here are my questions, would what I have done been enough to kill the bacteria and or ich? I think they had ich or velvet.
Despite rinsing the aquarium out in the bath with a shower head for ages, it still smells like bleach, what do I need to do to remove all the bleach?
Once all the bleach is removed Do I need to do anything else to make it safe to add new fish for quarantine, I dont want the fish I plan on getting to get a disease from this aquarium. I do plan on leaving it dry for ages after getting rid of all the bleach hoping if anythings left would die.

Thanks for any advise.

 

Fill the tank with water and put a load of dechlorinator in it and let it sit for awhile. swish it around to get it under the lip and down the sides some. See if you still smell it after that. Dechlor takes out chlorine. Chlorine bleach. It may just be some residue on the silicone or something.

 

I was going to say exactly what stargirl said. I wanted to add that I’ve read that bleach works better with cool water, but not sure if that’s true.
I’ve always filled tanks (second hand usually) with bleach water and left them overnight. The next day I’d rinse and rinse until I didn’t smell bleach anymore and then I’d rinse more. Next I’d fill it with water with copious amounts of dechlor as stated above and I’d leave that overnight too. It’s the chlorine that’s dangerous and this way it’s neutralized even if for some reason there is some left.

 

The smell was hydrogen sulfide. Perhapse your substrate was too compact or too deep. It's common with sand and dirted tanks. Also most of the bacteria you saw was anaerobic bacteria that produce said gas. Bleach is great for ridding pests like snails or worms, but it's a bit overkill for bacteria. Hydrogen peroxide works great for bacteria and doesn't leave a residue like bleach does.

 

Thanks for the help, so I will let it dry for a few days then fill it up with water and add prime. I assume forgetting to bleach underneath is unlikely to have re contaminated the whole aquarium again? when I first filled it with bleach water I used warmish water I used the hot water to rinse the bleached spray off because I realised I forgot to bleach the base so I hoped it would kill the bacteria/diseases if anything had gotten there.

 

Theres another issue, I need a quarantine tank but now I noticed the bleach linger Im worried about something else that was added before the bleach, there was white residue on the glass that wouldn't come off, no matter how much I rubbed, my mum has added some toilet limescale remover harpic I believe, is there anyway to ensure the chemicals are removed from the silicon or do I need to throw this aquarium away?