Might be a good idea as he is going to have a sore eye and things are going to look funny to him. He might not be sick technically but he could run about trying to figure out what is wrong and damage the eye trying to figure it out by running into to something. He will think it is almost bedtime and rest if it is darker. So good thinking to keep it darker. Daylight in the room won't hurt but I would not put any artificial light on in the tank.

You might want to get a divider for the tank you have the aggressive betta in and see if you can keep that little spitfire seperated from the rest of the crew to keep the damage to a minimum. It is a cheap fix for a possible problem. Also it may be a good idea to eventually invest in a reptile house like this one to keep on hand for an emergency hospital tank...it is what I use now with a sponge filter that I keep bubbling away in the back of my main tank. If they have anything contagious or that I do not want to put the sponge filter back into my main tank the sponges are easily replaced and I simply bleach sterilize the main part of the filter and pop a new sponge back on it and put it back in the main tank to reseed itself after it isn't needed in the hospital tank anymore. It keeps a permanent supply of cycled filters ready for sick fish and all that you need on hand is a cheap sponge filter some tubing and a cheap airpump (I use the whisper ones as they are super quiet) and a control device to control the air flow (also the cheap kind). Since I put the airpump above the level of the water line I do not need check valves but if you are going to put it under the water line you should probably also put a check valve in the airline for safety sake.

Pla-House Cage - Rectangular - Large | Terrariums & Stands | Reptile Cages, Terrariums & Accessories | Reptile - ThatPetPlace.com

they come in a lot of sizes but this one is large enough for most single fish to be medicated in.
I have a jumbo size that is a quarantine tank for new fish that come in until they can be introduced to my main tanks. They are easily cleaned and do not take up a lot of room. The lid at the top allows for easy access and does not allow for jumping fish.. the only drawback is the cord situation. I find it difficult to deal with cords so cut out places for cords but it takes patience.

A container at the dollar store will work as well if you can find one that is clear and in the size you like but you will probably not find one with a perforated top so you will need to make the top out of craft canvas or something ventilated.

Rose