Can anyone solve a mystery for me?
I have an established 126L aquarium which has been home to four rainbowfish, 2 silver tip tetras, 1 cardinal tetra, and a green goby for the last 4 years. Recently changed sand to gravel and had a big clean but kept filter running with fish throughout and in about 50% of water. Water parameters all OK waited 2 weeks and then added 2 baby red rainbow fish and 5 new cardinal tetras. All Ok. waited another 2 weeks and added 4 more cardinal tetras and 2 swordtails. Complete - or so I thought! 4 of my cardinal tetras have vanished under cover of darkness. No bodies, no signs of illness, water still fine. Who is the main suspect do you think? Only been disappearing since swordtails added but they dont look big enough to eat them. Two of the rainbow fish are a good size but they have lived peacefully with solo cardinal for years. Perplexed!