Hi Chenise' Mom and Welcome to the beginners section!
Pressure washing would just put undue pressure on the glass joins I believe and I don't think would help much with limescale. In fact, mineral deposits on glass, limescale, are notoriously difficult to remove, especially without damaging your glass. Glass is more easily scratched than one thinks.
There are people that get really, really serious and use hydrochloric acid. But this is quite dangerious and requires PVC heavy gloves, eye protection and special clothing and shoes even, so its rare anyone goes that far.
Going down a pretty big step on the acid scale, we come to White Vinegar, a nice bottle of which is usually easy to find at the grocery store. This can be used by turning the tank on its side and possibly using something to corral the vinegar into a shallow pool on top of the mineral deposit areas and leaving it overnight. Next day, move in and gently begin working on each small 2 inch area with some sort of non-scratch materia (3M make some sponges with this sort of surface.) This may take you 4 nights for all the tank sides.
Single-edge razor blades can help but you have to develop the right touch. The corners of the blade can easily dig in to the glass and create permanent scratches that basically look as bad as the limescale did. So you have to be patient and "feel" what the blade is doing. As with the non-scratch sponge, the limescale is going to come off much more easily after a night of submersion under the white vinegar.
Another trick is to use fresh lemons or limes as one of your rubbing steps prior to a more abrasive step. By taking a quarter of one of these fruits and pressing it againt the glass, the acid of the fruit juice will begin to work on the mineral bases and the inner surface of the peel forms a non-smooth surface to rub against the limescale and work on it. By coming at the limescale with a different type of acid, you may be loosening it up differently.
Sometimes people are selling or throwing these types of tanks out precisely because they've tried and tried to clean them, with no success, but if you are patient and think of it as a "project" you can sometimes be the one to have success and of course save yourself a pile of money over a new tank. Good luck!
~~waterdrop~~
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