If the heater was new it pays to check that as well, in particular smell the suction cups and see if they have that new rubber small. I bought 3 heaters in a matter of a months and my fish nearly died on several occasions because the suction cups leaked something toxic into the water. I did notice a distinct rubber smell when I got them so I gave them a real good rinsing with water, let them sit in the water and then re-rinsed but it wasn't enough. The fish became lethargic either laying on the bottom or floating on the surface not moving and their fins were VERY clamped!
I ended up having to ditch the new suction cups and luckily had a few spare old ones from my old heaters which no longer worked and then they were fine. The only thing which kept my fish alive during this experience was major water changes 50-100% 12 hourly changes. One night I got up as 12am and noticed how bad they had got through the night no I did yet another emergency change at around 12:30-1:00am.
I had another poisoning incident occur only a month or two back. My fish became suddenly lethargic with the same symptoms as mentioned above when the heater poisoned them only this time there was no new items in the tanks. It turned out to be our rainwater tank causing the problem. The tank is plastic and as the water level got down and the tank heated up in the sun during the day toxic chemicals leaked from the plastic into the water and that is what was poisoning my fish. It took a lot of trial and error to work that one out!!! I lost two fish as a result :( The scary thing is we drink that water....