| What fluorescent type is it supposed to replace?
I ask, because many have arc voltages around 100V and many even above that. These would be nearly impossible to feed via just a resistor in a reasonable manner (it will be way too sensitive for voltage variations). There are LED designs with linear dropper, but these use an active constant current source (usually a JFET with resistor in its source; these components use to be cryptically labeled as "LED constant current source", but they are nothing else than a power JFET, with the Idsat controlled).
With a resistor or series reactance the LED voltage should not be too much above 50% of the the circuit OCV to keep the current fluctuation under control, with inductive ballast it could be up to about 70%.
Because the limit for reliable discharge operation is about max 50% of OCV across the load, such ballast matches the LED needs with quite some margin (using higher LED voltage would mean higher power for the LEDs what goes against the usual motivation for LED in place of fluorescents, lower LED voltage than the voltage of the fluorescent may damage the ballast, so it is never designed that way).
That mean when the LED is designed to replace a 100V fluorescent, it may have voltage drop of 100V or above, so need a step up transformer (part of the ballast's functionality) to ensure stable operation, so wont work properly with just a resistor from 120V (resistor is not able to step up the voltage).
If the original tube is some 60V or below, you will have huge losses on the resistor (nearly double the power delivered to the LEDs), so quite missing your original goal...
If the tube voltage is in the 80..100V ballpark, you may get away with the resistor (so consequent current variation) if you make sure the LEDs, nor the resistor get overpowered at voltage corresponding to the maximum mains tolerance (you would need a variac able to step up to verify that). The drawback is, most of the time when the mains is around its nominal value, you get significantly lower power, so light output (could be as low as 50% of the rating). If you are OK with that, it could be your way to go, for LED lifetime it would be only better situation (unlike most discharges). But dont attempt to "fine tune" the power to the rated power at typical mains, the voltage will get higher for some parts of the day and fry your setup.
If the tube voltage is in the "resistor friendly" range, you may reclaim the losses in the case your tanks need heating anyway, so the ballast resistors could then be in the form of additional immersion water heaters (set a bit higher than the main ones, so they wont shut down on temperature; but not too high, to prevent overheating the living things in the water there in case the ballast power will be enough in some hot days). Their heat contribution would most likely not be sufficient to heat the aquariums alone (except some hot day, then the "ballast" thermostat will act and cut out the light), so the temperature will be still under the main heater thermostat control, but the main heater thermostat will throttle down the duty ratio so the heating power, so no extra energy consumption.
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