Push the packing tape down on the old adhesive with your fingers, then hope it peels off when you remove the tape. Wont allways work, but at least it wont damage the tank like a blade or solvent might.
if its on the outside of the tank you could try paint thinner but it may be harmful to fishes, you could try peanut butter, or a sponge it with coke acola then wash the coke off with hot water
Oh. Normally that stuff isnt that hard to remove unless it was exposed to an environment that made it bond to the acrylic. Ianab had a good idea with the packing tape. You may also want to try a warm hair dryer to see if that will soften it a bit.
Don't use paint thinner as this will cloud the acrylic. I had an acrylic window manufacturer tell me to use mayonnaise to remove old decals from acrylic insulated windows I had installed. It sounded crazy but, it worked.