Two reasons - the m/f from broods and health; too warm they die faster too cold they die faster. I suppose they aren't as sensitive as i feel they might be - though there are fishes where 2 degrees make a big difference.@anewbie
I have a question: Why do you always chase temperatures? I mean... The fish come from a vast area but except the Llanos species from Colombia and Venezuela, hillstream species from Peru and the southern species from Bolivia/Argentina the differences are really marginal between Negro, Madeira or Tapajós.