We got this frog about 8 years ago, and after about 6 months, we got another who turned out to be a female. They ended up with a daughter and son, and the four lived together quite happily. But one by one the frogs have died over the years, and a week ago, the son died, leaving the original frog by himself. Last night, he started acting very strangely -- hanging suspended in the middle of the water vertically, then going to the corner of the tank and popping up and down rapidly so his head was coming out of the water. I've never seen him do this before. His tank has both a filter and an aerator, and I replaced the filter fairly recently. After his weird behavior, I changed out half the water in the tank and put in new water. As of today, he's acting more normally.

Any idea what's going on with him? Was the water the problem, or is this some kind of loneliness behavior?

I'd appreciate any opinions!

Thanks,
Charlie