I made another post about it in a different thread, but long story short:
I just returned from a semester abroad to find my tank with no fish (Caravaggio is hopefully alive at someone else's home, he was supposed to be picked up over a month ago but wasn't) and a TON of calcium build-up. Apparently despite my plants and snails, my aquarium's caretaker saw no reason to have the filter on or do water changes (or even add water).
I was really overwhelmed at first, but then rolled up my sleeves and did what I could. Most of the original water is gone now and I've rinsed most of the substrate, so there's about an inch/inch and a half of water above the substrate (I have a 10 gallon tank).
I wet a paper towel with vinegar and pressed it against the calcium. After that I scrubbed as much as I could off with a toothbrush (the back wall of the tank still has a bunch but I just couldn't get it off). I wiped/scrubbed off where the stains had been several times.
What should my next move be?? Can I fill it with water or do I need to remove all of the substrate and rinse out the tank? The trouble is that I'd usually use a hose for that but because it's so cold I can't really do that outside.
My surviving plants (most of them were dead :( only my moss balls and a bit of java moss survived) and my snail are safe and sound in a half-gallon jar until I retrieve my betta/the three gallon tank he's currently in. I'd really appreciate advice on how to fix this after coming home to such a mess. I'm not looking forward to cycling the tank again, but will start tomorrow depending on how much more work I have to do on the tank.