I have recently come back from holiday, and whilst I was away i visited a fish store and asked for a heater for a 2.5gal fish bowl- the lady sold me a Hydor under gravel preset heater and told me that it was good for up to 20L
(this was also stated on packet) I had asked her up to what temp does it go at which she told me it would be suitable for that size tank.
knowing that bettas like warmer water 72-78, i thought this would be ok but after setting it up and running it - it onlys heats the water to 60-61F
He is not so lethargic now,but I am hoping he will be ok for a time till I decide to get another.
also tested water and found the following:
I do 100% water change every week