I've got the humidity quite high
The optimum humidity is 50-60%. You do not want it too high or you will
have problems with fungi (ie mold).
I've just sprayed the soil with some 3% Hydrogen Peroxide.
The best safe treatment for fungal infections indoors is bordeaux mix
(aka copper sulfate). If you have fungi in the soil, you are probably
overwatering and have humidity too high. Buy a moisture meter that tells
you when to water ($10 at walmart). Also keep a fan on the plants to
bring in fresh air and keep humidity down.
other than a high pressure sodium or a metal halide light, is there anything else that would actually make a difference to my plants indoors
Just normal fluorescent lights will work. You don't need the special
"plant" bulbs, but you will need lots of fluorescents to do the job of
one HID. If you want to use fluorescents, buy a 48" fixture with
electronic ballast that is made for T8 tubes. This will use less
electricity with same light output. The good thing about fluorescents is
that you can suspend them very close to the plants without burning. It
might be better in the long run just to buy an HID. Also the HID should
produce enough heat to keep your plants warm in the winter. Or just use
a small space heater. Put some plastic sheeting down under the garden to
help with cleaning and to help keep insects under control. Spraying the
plants with water every morning will help to keep mites under control.
If you do have an insect breakout, you can control them with neem oil.