I'm thinking that fish is sick... maybe ith whirling disease. It happens a lot with livebearing females. I have never experienced the disease personally so hopefully someone can chime in on helping the situation.
Whirling Disease as far as I can tell only effects salmonids in coldwater. The cuasative agent Myxobolus cerebralis hitchhikes into the fishes system through Tubifex tubifex.
I guessing that this is a bacterial or viral infection attacking the neurons. I'll see if I can find out more doing a literature search.
this disease does not only affect the salmon and trout of cold waters. It's something that also affects livebearers.... try doing a literature search on whirling platies, it seems pretty common.