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.
Yea i guess - i actually suspect a range of 10f long term is more signfiicant though short term probably not. Not to nibble here - but at least observationally i find there is a temperature where the fish is 'lively' but will live longer before it goes down hill (talking about long term).While it's right not to underestimate the impact of temperature, you seem to imagine smaller differences to have more impact than they actually do have. In many cases only a difference of more than 5 °C (roughly 10 °F) from an ideal average (usually 25°C) has actual impact while 1-3°C will not really have any impact. Even longterm.
When it comes to sex ratios in fry, ok, much different.