RIP Fishies

I got woken up this morning by my 9yo telling me that out fish in the fish tank where dead. sure enough they where. We had gotten the fish 3 years ago and i had taken the approach of not to much fussing with them, occasionally and i mean very occasionally I would empty out half of the water and top up with fresh and clean al little algae of the sides. but last weekend the tank was looking very dirty on the glass and the gravel was dirty and the water not so clean and the fish food had run out, I went off to my local pet store and enquired about a complete clean out of the tank. They advised me this would be fine and to use a sponge to clean the tank, rinse the gravel in clean water, put in clean water with the water conditioner and then put the fish in ther plastic bag on the top of the water to acclimatise them to the new temperature of the water in the tank. I did all this and put new plants in the tank. This process i did on saturday. they where fine yesterday morning being tuesday morning, then my youngest had said they need feeding and the old fish food was empty so i tole her to use the new one. Then we come to this morning when all 4 fish are dead. As we are getting a puppy in 2 weeks i will give the tank a break in the garage for a while before we get any new fish, I also wonder that maybe my decision to be really good and clompletely clean the tank was what killed them.