Temperature
Keep them at 70 to 79°F, 21 to 26°C. A seasonal swing inside that range is fine, and a warm spell toward 80°F can set adults courting.
Colder water is survivable but costly. At 17 to 18°C (63 to 64°F) redtails live, but eat less, slow down, and pick up more infections.
pH
Wild redtail water runs from pH 5.5 to 6.8. Stable, clean water matters more than hitting a number.
Oxygen: the quiet one
In low oxygen a redtail can keep eating and look normal, then stop digesting and die, without ever gasping at the surface. A big fish in a big tank needs strong surface movement and aeration even when it looks fine.