Well seems we all do it, eventualy I suppose. Get complacent and don't clean something enough or think "she'll be right" or just don't think at all and put something in a tank that later comes back to bite us in the tush. It seems it was my turn to do that yesterday and today I woke up to a 4 footer with 95% deaths.
My cichlid tank has now been reduced to a dozen electirc yellow and blue zebra babies (the smallest of the small) and the biggest of the big one humbug catfish (fingers crossed he'll make it). I've lost all my Kenyi, Aratus, my Bumblebee's, the larger 4-6cm baby electric yellows and blue zebras, and 2 red tailed sharks.
Yesterday I decided to top up the cichlid tank's sand with the sand from the 3 footer that I'd recently stripped. I was having a few die off's in there so moved all the fish to the 2 footer (numbers were suficiantly thinned to do this). I didn't think about the sand making them sick. Also I dropped in a courgette into the tank, from the garden, and I don't remember if I gave it a wash or not. I suppose this will be a costly reminder not to get complacent when keeping fish.