I have this 25l plastic fermenter which is the type not shaped like a big open bucket but with a hole at the top with a screw on lid. I left it full of water not thinking anything bad could happen to it for a few months, and I see now it has a dark mold at the bottom and other type of mold in spots on the side. I'm wondering if I could add a bleach solution to it, then empty it and add clean water and then use it again for brewing, or if it would be impossible to do without bleach being absorbed by the plastic and giving bleach off tastes in future batches etc?
Annoying problem is because I can't get my hand in there like a open top bucket type fermenter its almost impossible to manually scrub it etc..
If it is convenient, it may be best to just purchase a new one. Since the vessel is not clear you may never know if you were able to extricate all of the mold properly, and a chance at ruining a batch of beer is more expensive than a new carboy, in my opinion.
I don't recommend using it again. Plastic is fairly porous, at least compared to glass or stainless steel. It may also have tiny, microscopic scratches from past usage. There is no way to effectively sanitize the millions of tiny pores and scratches in the plastic which most certainly contain mold spores. Your bleach solution will not be able to penetrate into the pores, as well, you can not clean these areas mechanically (with a brush or scrub pad). Although it may be possible to "clean" a moldy bucket for something like storing dog food, or toys. It will never be sanitary enough for you to brew with, and most certainly for you to ferment in. I strongly recommend that you recycle the bucket.
I've never had to clean mold, but I ferment in plastic carboys (primo water jugs) and the krausen comes off with an overnight soak of oxyclean in hot water. I've never used a brush, so no scratches to worry about. All visible gunk comes off with the soak. Before you throw it out, you might try that.