
I got Eheim/Jäger brand heater, it's 100W and said to be good for 100-150 liter tanks. And my tank is 128l and holds average temp of 25,5C, with heater only set to 21,5C(and it's not on most of the time, though usually it kicks in at evening/night). I've also read if you need to heat up the water much higher(I can't remember how much higher...) than the room temp, you might need a heater that has little more power than usual. So yes, sometimes higher powered is better. ^^
I've seen similar injuries on few of the fish(ottos) I've had that have died, both were okay when I had last seen them alive. But when I found them dead one had tail missing and other had been nibbled little on it's side... So I'm guessing when they died other fish came and see if they are edible...

I got this plant called Limnophila sessiflora, that my tetras seem to favor the most. It doesn't need ton of light and grows like a weed with right conditions(said to be good beginner plant as well), so at worst I've had to trim it down once a week.(though now it's not doing so great, and I'm not sure why...) My tetras also like to hide behind the large tree I got in the tank. Well, they generally like to spend time at the back wall, so they are almost always out of sight.

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What I find my tetras doing when I come home after 10pm, if I've forgotten the other light on...

And I'm giving another shot on growing baby corries, this time small amount and all from same spawn... I got 8 fry now(out of 9 eggs, so not bad hatching rate), all hatched couple days ago. One of them hasn't looked as strong as the others from the start, so I might be down to 7 soon.