I need urgent help - I'm about to loose another fish


I have a 95 litres established tank - it contains wood rock and plants ( lots of plants )

over the past few weeks I have had a recurring problem that I cannot find the solution to and tonight I am about to loose my Sailfin male.

in short the scenario is this ........ every 4 - 5 days a fish will go down and when I say go down I say literally with hours. it is always the same sequence - fish is fine .... fish sits on the bottom - fish leans over - fish dies

there is no consistency with the type of fish - Ive had Guorami's, sword tails, tetras ..... the perplexing part is that I trend to have a male and female of each and whilst one will die the other is fine - my sick sailfin is the male for example but his pregnant female of perfectly fine. the sword tail was female - fine at 10.30 by 4 pm she was dead - the male is fine

I religiously test the water every other day and it is fine - I have taken it to various aquatics centres to be tested in case I am missing something and they cannot find anything wrong with it. I do regular water changes of 20 % with the appropriate Tap safe added. the water in the tank is perfect.

the temperature is a constant 26 C ( I have a digital prope thermometer )

there is a tank length aerator and a long block one too

I have removed the snails and they are presently living in another tank

I had in the tank

6 x Tetras
2 x honey dwarf gourami
I flame dwarf gourami
2 blue gourami
2 red dwarf gouramis'
2 sail fins
2 sword tails
4 small rams
2 small rosie barbs

I have lost the female sword tail, both blue gouramis' ( which died weeks apart ), both red gouramis', one tetra and now the male sailfin.

the fish have come from different outlets
I have added salt

I added melafix - made no difference

Esher 2000 ( having removed everything previously with active carbon filters )

the fish that die have no sign at all of any illness - they do not have any damage to fins or tails - I have had a 'scrape' done on a recently dead one with negative results for anything

they are fed tropical flakes and once a week either blood worms or shrimp lavae - both clearly state they are suitable for all tropical fish

I'm at a total loss as to what is going on and I do not want to loose any more fish

Help !!!!

thank you

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https://www.fishkeeping.co.uk/modules/ ... wtopic.php?post_id=348948