One of my adult bristlenose has suddenly appeared half eaten and rotten It really badly stank. Ive had it for about 18 months

Its that bad I cant work out whether its my super red or L144.

I don't understand what could have happened. All my other fish were gasping for air this morning and I had a dead molly and a buenos aires now I know why

Ive taken a bucket of water out and replaced it, only did a big water change last night so I cant really do another one. Not sure what else I can do?

You need to do some water tests, it sound like some kind of poisoning possibly Ammonia or Nitrite.

Did the fish seem OK before the big water change, if they did then what percentage of water did you change.

Is there any drop in filtration, change in temperature or change in pH.

For now I'd do some daily 20% water changes and use a product like Nutrafin cycle that reduces harmful ammonia and nitrite.

Yeah ill do some tests when i get home from work.

The fish were fine before I changed the water. I changed about 25%, im beginning to wonder whether something got in the water.

I always change the same percentage of water and never had a problem

Post the results of your water tests they will help with a better daignosos.

Any extra information like size of the tank, stocking levels, filtration etc will also help.

were the fish gasping for air at the surface or were they gasping for air mid-water? If its near the top then it might be that you are low of oxygen. either way I would be adding in an extra airstone (and def adding one if you dont have one!) Bristlenose are very sensitive to oxygen levels and would be the first to go if the levels were low.