The variation in fish price is astounding, the most expensive fish we could buy at our supermarket fish counter is talapia. Whilst an all fish diet isn't ideal, if it is for a short time it wouldn't hurt too much. Personally, I'd extend the feed intervals, maybe reduce the amount fed to each, and if I was group feeding I would be careful to ensure that one wasn't pigging out and others getting nothing.
You mentioned that you couldn't turn your heaters down much. You have thermometers to monitor the temps, right? Could you turn them off for periods to drop the temp and slow down their metabolism? Obviously you'd need to monitor the temps yourself rather than relying on the heater thermostat. Just a thought.