Regardless, dramatic change of environment will very often cause the buds to fall off or fail.
Some of the buds might still bloom.

Most Holiday Cacti die from too much moisture as a result of overly moisture retentive soil....
and usually, peat moss is the main culprit.

These are jungle cacti, which grow in the crooks of trees. They receive rains quite often.

I find that a mix of bark (and perlite, pumice, et cetera) is perfect for these plants.
It allows you to water as nature waters (which is to say frequently and thoroughly) without fear
of root-rot.

Josh

If the plant has not shown budding yet....and the outside temperatures allow it, put it outside to be subjected to cold...not freezing...in a protected place...say up by the wall behind foundation plants or near the garage door to give off reflected heat, then bring it indoors. The cold might just jump start the budding. If not, then leave it where it is and hope the light will trigger budding.
Some C.C. bud at this time....Thanskgiving--through Christmas, die back some, then re-flower later in January/February when light levels increase.

As far as watering, only water when the plant tells you to.
Stick your finger in...if it feels damp, let it stay so for another few days, then test again.
When you water, water to drainage and dump the excess...never leave water sitting in the saucer below for more than ten or fifteen minutes.
Never leave a C.C. in a wrapped with paper condition. The paper doesn't drain and builds water below to be re-sucked up to the roots which soon cant take up anymore and root rot develops. The lower leaves is the first sign of this happening.

I have a couple of fish tanks that require partial water changes twice a week, so I usually use the water that has been removed from the aquariums for watering my houseplants. I figure that the fish wastes may act as a mild fertilizer, and most of my houseplants have in fact started doing a little better since I've been following this regime. But could using fish-tank water cause problems with fungal diseases? The water is always at room temperature or a little warmer.

Just wish to clarify, it's not the POT that makes good drainage. It's the mix, that's what matters.

Is your mix hardened off (& therefore breaking plant stems at the soil line)?

I disagree w/ the above poster giving you all this extensive care instructions about bloom without discerning what's actually wrong w/ your plant, or if it's even if going to make it that far. Cart before the horse in my opinion.

I'd forget fungal diseases & just worry about plain old ROT, I wouldn't give these plants fish waste water, too rich.

I'd like to suggest you forget the supplements, fancy pots & fertilizers & work w/ basic soil mixes. Unless one gets the mix corrected, all else is likely a waste of time & money.

I really don't know whether the planting mix in my long-dead previous Christmas cactus plants had hardened off or not, but it is certainly possible. I'll try to keep an eye out for that this time around.

'They' implies you've had more than one Holiday Cactus. If each plant is undergoing the same problem, your Holiday Cactus don't have fungal disease.

Don't toss it. A picture would be helpful.
Since there's no picture, it's difficult determining the state of its health. If your plant is in in ill-health, it's too bad you couldn't start out with a healthy specimen and follow advice.

I agree with Golen.
Didn't see if he/she mentioned there are two ways to force Holiday Cactus into bloom. One is by giving it short days or second, kept outdoors until first frost. I go the second route.
Short day treatment can be a disaster since timing is very important. A few minutes late can make a difference. It's much easier leaving outside and letting Mother Nature do the job. lol

Both soil and pots are important.
Too large a pot can halt flowering. Roots should be tight-fitting.
Pot material matters too. Some people believe plants need daily watering. Plants will die from over-watering, especially in winter.

For those who must water daily, clay pots are a better choice since soil dries faster in clay. Otherwise, plastic pots are sufficient.

Also, pots MUST have drainage holes. Without drainage, the top will look/feel dry, but builds up in the bottom half, keeping roots constantly wet. The results, root-rot.

Since Holiday Cactus prefer acidic soil, I add a little peat in the mix. However, well-draining soil is a must. There are several medium/soil recipes here on GW and Google.

Fish water is okay, but high in Nitrogen. Perhaps alternating with Flowering Plant food would suffice.
A balanced fertilizer with additional nutrients will cover all its needs.
Fish water works great with foliage plants. Plants known for their leaves instead of flowers.

You also mentioned your aquariums are partially cleaned, twice a week? (isn't that a bit much?)
So, do you water with fish water twice a week, too?

Unless the soil is thoroughly dry, there's no need for two waterings per week.
If you've been watering twice a week, and your Holiday Cactus are shriveling, most likely you're over-watering.
Check soil before giving a drink. Stick your finger or a stake deep in soil. If it comes out wet/moist, wait a few days and retest.

Holiday cactus prefer medium, bright light in summer. During winter months, place in your sunniest window. Unless it's flowering, then keep in semi-bright, non-direct sun.
A cool room is preferrable. Too hot, buds and flowers drop and/or fade faster. A bright, enclosed porch is ideal.

Good luck. Please don't toss your plant. Toni

I'm sorry if I was unclear but now I'm confused. I'm not asking about your long dead plant you mention, I'm asking about the one you have now.

It's easy to tell, when you turn the pot out upside down, is the whole thing hard like a rock, including the mix? If so, that's what the problem is, the mix is hardened & the water is not penetrating the rootball, rather likely flowing down the inside of the pot btwn the hardened rootball & the pot's wall. Or, feel the rootball w/ your fingers, is it hard to the touch?

If this is what you find, pls. come back & confirm that & then I'll suggest how to proceed next to try & save the plant (if you want to).

Without correcting the mix problem, it's unlikely you'll get more bloom.