male betta died

had him and a female in a 10 gal. for about 6 weeks. yesterday he started
looking lethargic and this morning just kinda drifted off and died.

everything for the tank is normal except that the temperature runs about 78
F at night and 84-86 F during the day (no air conditioning in our house).
been keeping the lights off on all the tanks, lids open, water circulating,
air in the house circulating, everyone else is fine and the larger tanks
fluctuate less in temperature than the 10 gallon.

preceding his death he was acting normal, flaring, chasing the female,
building bubble nests, eating, etc. only other abnormality in the tank was
the african dwarf frogs perching on the plants at the surface rather than
hanging out on the bottom, this was about three days ago and was the only
time they did it.

other occupants of the tank; 4 ADF's, 1 female betta (who is doing fine), 4
danios (these were feeder fish that my wife kinda likes so we've kept them),
and 2 ghost shrimp. (as a side note; i'm trying to convince my wife that
*she* needs a bigger tank to keep all these guys in)

as i mentioned, all the parameters are normal, no changes in the chemistry,
2 gallon water change per week refilled with 2.5 gallons (evaporation), i do
a w/c in this tank every week because of the higher load, only thing off is
the fluctuation of the temperature.

idea's or suggestions?

thanks,
tedd.