i had one for about 5 years and replaced the ballast just before i sold it. well, when i say "I", i actually dropped it off to a sparkie mate who fixed it. He replaced the ballast and didn't charge me anything at all so figure they aren't expensive. One tube wasn't working, when i swapped the tubes the same side failed to start so it's the ballast.
don't be worried about sunsun tanks, they are far better than the aquaone units in every way IMO.