Added 3-month old raised Pearl Danios to a tank containing Barilius and Humphead Glassfish, lost half the group in <24 hours.
Adding new Synodontis nigriventris additions to an existing group and moved Steatocranus "dad" to same tank, rapid case of Ich wiped out my Synodontis decora youngster group; dad; one of Synodontis nigriventris.
In middle of ~75% water change for my Aphanius mento, then realised I did not have enough coarse sea salt to get anyway current levels (while treating illness), decided to use a mix of Epsom salts and low sodium salt as a temporary measure (used before with no issues), except this time the whole group keeled over during the night and following morning.
Added 8 Ilyodon xantusi youngsters (~4cm) to my tank of ~50 (~5cm) Steatocranus youngsters (adult goodeids were fine and great dithers in there already), one was pulled apart in seconds and I somehow saved the others.
Put an oddball ~5cm catfish in my bogwooded 620T, then discovered it was a Phyllonemus spp. (suspected filinemus) from Tanganyika, then found out the hard way that they really do not do well in water holding organic matter like lots of tanins (despite water being perfectly hard for Rift Valley lake fish).
Exposing a group of Aborichthys elongatus to ~28C water during the heatwave of 2010, lowered the oxygen level too much, only one survived and is still with me today. Not sure how I could have done something positive here, everywhere was hot in our flat and garage for those few weeks.
Underestimating the psychotic nature of breeding Steatocranus casuarius, killing tankmates even in a Rio240, including African Butterfly Fish; Humphead Glassfish; Golden Wonder Panchax.
Removing breeding Steatocranus to a nursery tank, all seemed well, then without prior warning male kills female (he wants to breed and breeding aggression is unleashed on partner if there is nothing to protect fry from... but usually there are warning signs for a few days, where mum treads water in a top corner submissively).
Using the "painting a rock" with egg white and crushed food for new Rubbernose Plecs, except I went bonkers with the quantities and the "cocktail" was not as dry as it seemed... Phonecall mid morning from better half saying tank water is cloudy with an awful smell and foam on the surface! 4 ~95% water changes in 3 days (using the only ever one from my better half when initial problem was spotted), a miracle that they all survived!