Vinegar helps. I use lime remover (any brand) from Walmart/Homedepot. Works much better.
Shouldn't hurt the glass , but try in a spot on the back of tank at bottom just to be sure and do a small spot on the light cover . It may scratch plastic .I was wary of using harsh chemicals, but I assume with enough rinsing then these could be made fish-safe?
We have Barkeeper's Friend under the sink, so good to know. Is it not too abrasive for the glass?
Forgot to ask originally: can any of these cleaners be used on light fixtures and similar electronics? Somehow the buildup even got on the plastic shield for the neon tubes (there was a glass canopy in between...)
I try not to go any stronger than works well so I would give vinegar a first try before moving to stronger. If there are really big lumps, there are a couple other things that may help. A scraper like designed clean paint off windows will be a real time/work saver. these come in dozens of types and designs but I like those which use single edge razor blades. I find that I get much better value if I keep a fresh blade as it gets down to get under the really thin layer at the surface. Then after scraping, soaking paper towels in vinegar and laying them on any tough stuff will usually be enough.
Some chemicals that I have used will leave a bit too much residue and effect the PH at times. Acid Magic is one that does a great job but it also gives me fits to get it all off. Vinegar is great for most stuff. I once bought a repot house with tank and dead pleco carcass!