I have a 100 gallon long tank and am currently using a 200 watt Eheim Jager heater. What wattage and how many would you recommend for this sized
tank?
I would probably go with 2 heaters..
wattage will depend a little, at a high level, you don't define your need for a heater by the volume of the aquarium, but more the volume of the aquarium and the temperature change you need.
A heater will raise a certain volume of water, to a certain number of degrees above room/ambient temperature; There will be differences between brands for things like efficiency so they will not all be exactly the same number of watts per gallon but they should be close enough for our needs.
looking at eheim, I can't see anything documented (easily) for how much water their heaters will heat to a certain temperature.
So, if I look at Hydor Theos, and look at their manual,
You can download here, at the top right there's a small chart showing ∆t of 14°F for each heater, and tank size.
so say you want to raise the temperature from a room temperature of 70°F to 84°F, the chart says that a 400W heater will do the job. or a 200W would do half the job, so you could use 2 of those instead.
The reason for choosing multiple heaters, is in the case of failure. In my example above, if you have a single 400W heater, and if fails OFF, your water would stay at room temperature; If it fails ON your water would be 14°F above ambient. If you choose 2x200W heaters and one fails OFF, you could be 7°F above ambient at most; If one fails ON you could be at least 7°F above ambient (The other heater would continue to work if you needed more)
now, if you think about change in ambient temperature, say it can go from 65°F in winter to 80°F in summer. worst case scenarios are tank temperatures of 65°F or 94°F for a 400W heater, or 72°F to 87°F for 2x200W heaters (Assuming only 1 failure)
My personal preference would be for 2 heaters, each rated a little higher than half of what you would choose for a single heater. You need to know what temperature you want your water, and how much your room temperature can change throughout the year.
TL-DR - the more heaters you use, the less chance you have for extreme low or high temperatures, using 2 or 3 heaters will lessen the chance of catastrophic failure, and also allow you to safely use more overall wattage which will allow things like water coming up to temperature more quickly after a waterchange. If you want a quick out of the box answer rather than checking into your needs, I'll say that 2x200W Hydor Theo heaters would meet your needs.