So I'm not new to fresh water aquariums, the tank in question has been setup for several years. Main tank is 750 (ISH) litre's with the sump about 350. My main fish are my 6 clown loaches and 3 bronze croydrois (bad spelling) all of which I've had for 5 years or so. Along with that I've a few different tetras and an abundance of sword tails. Too many to count. Have never had issues before apart from them breeding. They all came from about 7 I bought so inbreeding could be the issue. Over the last few days I have been taking out about 6 a day that have died No white spot or anything of such nature. The odd one or two are flicking off the substrate (sand). I can not see any sign of white spot. I have some treatment on order for white spot and also have aquarium salt that I am now reluctant to use as the opinion on salt and loaches seem split so have not used it.
Recent changes ... I added 4 angel fish before Christmas to (hopefully) keep the sword fry down to a bear minium or with any luck zero. One died shortly after getting ( shock probably) the other 3 seemed un happy. On investigation I found the water temp to be at 19 given that I bought 2 new heaters and increased the temp to 25 over a few days . . That's when they started dying. The ages of the dead one all vary and are all sword tails.
They all take food and the angels seem happyier now. My fear are my clown loaches given I have had them so long. Water is changed regularly and is all in the right area across the board. I did notice one of the clowns flicking over the weekend but this morning they were all out playing as they do.