Waterbug, i feel like they do need extra fert as they have been slowly dying off. I have light on them 12 hours a day. Sometimes more.
I'm more of a test type person.
On the list of things that will kill plants not enough fertilizer is pretty far down the list, very close to the bottom. Lack of fertilizer is normally associated with slower growth, not as much growth as desired, weak growth, etc. But actual death...nope. At least by experienced gardeners.
For pond type plants you'd need a tremendous amount of artificial light and probably heated water. Way, way more than a couple of bulbs if you wanted actual growth. And the lights have to be very close to the plant, almost touching it. And reflectors to get light hitting more of the plant. And the lights would probably have to run 24/7. To our eyes it may appear that a couple of bulbs are pretty close to daylight, but if you put a meter on it you'll see it isn't. Plus the human brain changes our perception of light so we are very poor judges of light intensity or even spectrum.
Next is temp. Most pond plants can survive low temps at night as long as day temps are high. Cool water and cool air coupled with very low light and the plant sees winter coming and starts shutting down. Many pond plants store energy in roots, rhizome, etc., and want to move energy out of the plant and into storage before a frost comes. Your plants may not be dying, might just be shutting down. Once the shutdown happens it can be very difficult to trick it back into growing.