PlanetCatfish.com

Hey, I'm new to keeping fish, so I don't know if this is natural, but I wanted to check. My corys are a constant source of joy and worry to me, because they do all these weird things, and I don't know whether or not they are natural. My corys very occasionally do this weird thing: They swim around normally, then all of a sudden, they start twisting around wildly and flipping onto their side and back up again really quickly. Here are my tank parameters:
77-78 degrees Fahrenheit
7.0 pH
Hardness 40ppm
Alkalinity buffer 130 or so
Ammonia and nitrite both 0
Nitrate 10-20
Water change every week
Tank Set up
29 gallon
Fine black sand substrate
Penguin biowheel 350 filtration
2 echinodorus, 5 dwarf saggitaria, 2 lava rocks w/ holes,
one large piece of bogwood as furnishings.
Three bronze corys, one albino cory, one julii cory, one albino ancistrus, 6 black neon tetras, 5 cherry barbs (three male, 2 female).
like i said before, they go wild trashing around in the substrate. This only happens very occasionally, maybe once a month, and the fish usually recovers very fast. It is not always the same fish thrashing.
I didn't want to do anything yet for fear of making things worse, but I did put in some melafix to help one of my corys heal his fin after he tore it.
Please help, I already had some corys die from a fungus, and I don't want it to happen again.
Thanks!

has been set up 8-9 months

Your description sounds like they're flicking in the sand,which basically is either ick,although cory don't tend to suffer from normally... or they just have an itch,possibly scratching themselves,if they are only doing it every so often its nothing to worry about

My corys are always doing weird antics,tossing and turning and generally being nuts

Thanks a lot! I looked and it doesn't look like they have ich, but I'm going to watch them closely, my LFS has a severe epidemic of ich and fin rot going around. One poor little tank of panda corys had some fish in it who almost didn't have any fin left! It was really sad, but they were treating the tank, so hopefully it will get better.