It is very common for many aquarists to use used aquariums. However, there are some very serious risks associated with used tanks. They are unavoidable and the amount of water damage from a leak can cost a fortune to repair.
Do not trust used tanks. The cost savings simply are not worth the risk of a leak. Even checking that it holds water doesn’t mean it won’t split at the seams a week after you fill it.
Working in the industry for so many years has allowed me to see and talk to a lot of people who have had bad experiences with used tanks. Most stories that end with “…and all the water was on the floor.” start with “I got a good deal on craigslist…”
Is it only used tanks that leak? Absolutely not. New tanks can leak too. But if you buy a new tank can take care of it (use a proper stand, make sure it is level, NEVER move it at all unless it is completely empty, and don’t let it sit in the garage for years) you are unlikely to have any issues.
Do used tanks always leak? Absolutely not. Tons of people have used tanks with no issues, some for decades. But with used tanks you have no idea of you got one from someone who took perfect care of it, or you are the fifth person in a chain of people who moved it full of sand and four inches of water, didn’t support the entire perimeter of the tank on a homemade stand, etc.
In case you still aren’t convinced that used tanks aren’t worth it, here are some horror stories:
I had a customer who got a used 200 gallon on craigslist. He had it for a long time, well over a year, when one day his kids came upstairs covered in glass and water saying, “We didn’t touch it.” The entire front panel of glass came right off the tank and shattered. All 200 gallons were on the floor.
I had a service client who called me at 10pm because his 150 gallon was leaking. When I got there I saw that the seam had split, the top four inches or so of one of the corners just split. It was built into the wall and water spread out through his entire basement. He got his tank used as part of the house and had no idea how old it was.
I got a used 100 gallon and used it for a few years before selling it to my cousin. A few days after filling it one of the seams just split down the entire side and all the water drained out in a flat sheet of water.
I had a used 55 that randomly started leaking one day. It was on the third floor. It turned the living room in to a waterfall and even drained in to the basement. It was over $1,000 in repairs to get the drywall and flooring fixed.
I also had a used 10 gallon that randomly emptied its entire contents one night. When I woke up there wasn’t any water in the tank. Again, it drained in to the living room. Just ten gallons cost over $500 in repairs.
I had a customer who bought a 180 off of craigslist. He was very paranoid about it leaking so he set it up in garage for over a week. Six months after setting it up and spending over $1,000 on live rock and another $1,000 on fish (nice saltwater fish) the tank leaked out of the bottom and drained the entire volume of water.
A customer who was uber paranoid about leaks bought a 125, resealed everything, and tested it for two weeks. Sure enough a year and half later it leaked. He thought he had spilled water because saw some on the glass but when he looked close he saw it was a small stream running down the side of the tank, starting about two inches from the top.
These examples all show that used tanks simply cannot be trusted. Even when tested or running for months or more they can leak. Saving even a few hundred bucks on larger tanks just isn’t worth risking the massive amount of water damage they can do.