Yes the fish tank heaters will be perfect depending on how many gallons you are heating. They usually tell you what size fish tank heater they are for (5-15 gallons etc.) You can probably even use it for more gallons than that, as long as it's not way over like 50 gallons for a 15 gallon heater. Also most fish tank heaters that have pre-set thermostats are set at 78 degrees, that's too high. You want the adjustable thermostat models. The
nutrient solution temp should be between 68 and 72 degrees, can be a little lower but should be not higher. You can get fish tank thermometers for around $2. Although I wouldn't recommend the stick on kind they are hard to read and I don't think they remain accurate that long.