"DBB" wrote:

Its in a 6 inch plastic pot.
I am using a combination of potting soil and compost manure for the soil.

They do best in a mixture of soil and sand. They need very good
drainage.

I water that plant 3 to 5 days a week.
Light is provided by an aquarium light.

They do need light, but they don't need much water. In the wild they
may only get watered a couple times a year.

When the plant first started doing bad, I put a bunch of fertilizer tabs in
the soil to give it a boost.

Cactus have very slow metabolism and don't need much fertilizer. In
fact fertilizer may stimulate new growth which a stressed plant can't
sustain.

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.

That is a sure way to kill a cactus. They don't need much of any water.
Water will kill them. I have a dozen cactus in a room where they sit in
direct sun light and I only water them once a month. And if I am on a
trip, they don't get watered at all. Some cactus don't like water to
touch the above ground portion of the plant. It can cause rotting if
the humidity is too high. In the wild they grow in low humidity places.

The major killer of plants is too much water. The second major killer
is too much fertilizer. Cactus basically do best under the worst
conditions: sandy soil, seldom watered, seldom fertilized, full sun.

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