45 Gallon planted freshwater aquarium about 4 months old. 76 degrees with an Emperor 400 Bio-Wheel filter with a DIY co2 system.
Last weekly water change 3 days ago. Feed tetra flakes, frozen/freeze-dried bloodworms/brine shrimp, occasional vegetables (peas, cucumbers) and algae wafers. Have 135 watt 6700k bulbs.
PH = 7.6. Ammonia/Nitrite = 0. Nitrate < 20
8 Tiger Barbs, 3 Upside Down Catfish, 1 Raphael Catfish, 6 Corydoras, 4 Otocinclus, 1 Pearl Gourami, 4 Amano Shrimp
I purchased 8 tiger barbs (4 green/4 tiger) about a week ago. All seemed fine in the store. 2 of the green tiger barbs spend a lot of time (not all the time) with their nose pointing towards the ground and not swimming with the other barbs. Earlier today I saw one of these 2 lose control, roll over on his back and just sink to the bottom. It looked as if he was dead. When some other fish came close to him he snapped back up and returned to the school. This was about 3 hours ago. Everything seems ok again. None of the other fish are exhibiting this type of behavior.
What do these symptoms suggest?