In article uM0Be.12699$ao6.1208@trnddc05, says...
I just bought a used 55 gallon tank and it has a lot of calcium deposits
around the top. I would like to clean this off before I set it up. I tried
to scrape it off, but it is slow going unless I use metal tool and I am
afraid I will scratch the tank glass and the plastic moldings. I was
thinking about using a weak muratic acid solution but wanted to see it
anyone else had a better idea first. My wife thinks it will etch the glass
if it is too strong.

Anyone have a good solution for cleaning hard water deposits off a tank?

There are janitorial products for removing such deposits, such as "CLR" (Calcium,
Lime, Rust remover). I would use one of those because they are effective and not
difficult to wash away. Use it to wet an aquarium scrubbing pad for mechanical
plus chemical action. Perhaps avoid scrubbing the silicone since it is too soft
for the glass scrubbers.

At one time I thought muriatic acid would be just the ticket and bought a gallon.
It was completely ineffective at removing the deposits. My experience is with
acylic tanks. I have not tried any of these products on glass but I wouldn't
expect any problem with the glass or silicone. Glass is not easily etched, and
certainly not with muriatic acid or any common cleaning product.

The other tool that I like for this job is a power washer. Often the high presure
spray of water is enough to clean the tank without any chemicals. It is great for
a prewash then the final rinse after chemcial usage. Acrylic is not damaged at all
by reasonable use of the power washer. With a 6 HP model and 40 degree fan nozzle,
you can get 3-4 inches away fom the acrylic (including seams) without any visible
effect. I don't know what would happen if you went closer; perhaps nothing.