I find the worst fishkeeping deaths to experience are fatal accidents which in hindsight seem to have been preventable but were or could not be foreseen beforehand. A near-miss event which has just occurred, coupled with a fishkeeping video I happened to see recently of a well-renowned fishkeeper who herself experienced a seemingly unforeseen adverse event, made me think it might be wise to start a thread on people's experiences of these which others might learn from and so be able to prevent themselves.
Examples in my own experience, and potential solutions, include:
* fish which somehow get trapped between the back of the filter and the glass, despite the space seeming impossible for that to occur >
** (1) using plant-keeping tongs, I now tend to squeeze filter wool into any gaps around (above, below, at the sides, at the back of) the filter;
** (2) waterproof magnets for insertion in the filter and the outside of the aquarium glass [I had to contact a couple of magnet companies, who were very helpful in their advice and had not previously issued magnets for this purpose, and bought from there - but NB the magnets are extremely strong and they do understandably come with strong health warnings about potentially trapping fingers!].
* fish which inexplicably end up on the floor >
** (1) I have sometimes blocked up the cable holes with filter wool but this resulted in a pool of water around the tank resulting in me thinking I had a tank leak when in fact it was only the condensation spilled over the lip of the aquarium cover;
** (2) I am more careful now when removing decor at the start of a tank clean, by giving it a gentle but prolonged shake to release any inhabitants - one theory I had was that a neon green rasbora had been resting near the identical-coloured hornwort and hadn't time to disentangle itself to escape before I whisked the hornwort out of the tank.
* fish swimming into plant-keeping tongs or net or jug >
** (1) I tend to lower tongs into the tank in the closed position to reduce the possibility of fish getting between them. However, just now, I did so, only for a harlequin rasbora to dive between them just as I was closing them to lift out the offending item (a piece of fungused food) - this was somehow a near-miss experience, but I may need to re-consider this approach at all;
** (2) while some fish are fantastic at escaping nets or jugs, especially when the intention is to capture them, they seem to 'volunteer' to go in when the intention is to remove food or water respectively, so it's worth being extra careful to check before complete removal from the tank.
I'm sure that many other examples of mine will spring to mind later on, but thought this was helpful as a "starter for ten", and that others might care to add their own experiences and potential mitigators/solutions to this thread.