Hi. Bit of background. i have currently 9 Cardinal tetra in a 200l tank with 10 harlequin rasbora, 6 long finned cherry barbs and 3 large amano shrimp. I have had a spell of problems with my tank and weve lost 1 cardinal recently along with all the non-shoaling fish i had. I think weve sorted the issue by rehoming a very large pair of male white finned rosy tetra who i think had bullied half the tank. Initially my Rosies had been in a group of 7 but i have been steadily losing them over the last 9 months and didnt want to replace them. My water parameters have been stable and ive been testing throughout - ph is 7 kh 3-4 (i have to buffer to get any kh at all) nitrites 0 nitrates around 5 (very planted tank) and ammonia 0.
I have 2 cardinals in my shoal who are much smaller than the others and are very tatty finned. Am fairly confident its nipping rather than rot as ive seen one of the big cardinals have a go. Theyre very active and swim with the others but am concerned am going to lose these too. Is there anything i can do for them or is it usual for there to be a big size variation? Ive tried to get a pic but theyre fast! Im fairly new to all this we established the tank 12 months ago and first fish (the cardinals) went in about a month later after a fishless cycle. thank you
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& welcome to 
