A 300w heater in a 10g tank? I wouldn't keep it in there unless that tank was kept in a freezer. That sucker could make a 10g pot of soup.
If those are neon tetras in your 10g, it's said that they do better in cooler temps anyway; 68-74F. Cardinal tetras are ~73-80F
FWIW, I think the heater in my planted 10g is a 25w, and I rarely keep it plugged in. The tank maintains room temp in the winter. ~68F or so.