I have a small green cactus that has a red flower on the very top of it. It
almost looks like a cactus version of a rose. It is doing very poorly,
turning brown, sagging, and getting soft. Heres the conditions:

Its in a 6 inch plastic pot.
I am using a combination of potting soil and compost manure for the soil.
I water that plant 3 to 5 days a week.
Light is provided by an aquarium light.

When the plant first started doing bad, I put a bunch of fertilizer tabs in
the soil to give it a boost. When that didnt work, I sealed the hole in the
bottom of the pot so it would retain water better. I know plants need lots
of water to remain turgid.

My dad is never going to let me run his garden nursery if I cant even take
care of a cactus, help!!!!!