See attached images. I've had this 70-gallon tank for 5+ years and never seen this before. I have not added anything new to the tank in the last year or so and, in general, nothing has changed recently. I started seeing this a few weeks ago and didn't look very closely. I thought it was food flakes sticking to the inside of the glass. I took a closer look today and realized these things are living. Are these snails? What do I do?