There are a lot of places one can get a used aquarium: Craigslist, eBay, local newspaper ads, or even at swap meets at your local aquarium club. This is a great way to save some real money on a new-to-you aquarium. You can even find people who are just giving away aquariums for free! But, since the aquarium is going to become the entire world to your future aquatic charges, there are a few things you should consider about your new tank.
Many people use fish tanks as housing for pet reptiles. As such, these tanks are often cleaned using soap and water, or other cleaning products. Make sure the used aquarium you are acquiring has never had any soap in it! Soap residue can and will remain in a tank for pretty much forever, and even trace amounts are deadly to fish and other aquatic animals.
Older tanks, especially the all-glass style of aquarium, can develop leaks in their silicone seals. The aquarium pictured below is a tank I got from the curator of fishes from the museum at which I volunteered when he retired. The tank was made in 1991, which means it rolled off the line when I started high school.
To test the tank for leaks, which are pretty likely in a 24-year old tank, I set it out on my porch and filled it with water.
After it was completely filled, I let it sit for several hours. That way, if there was a slow leak somewhere in the silicone, the tank would eventually show it.
Happily, the tank is leak-free. If you leak-test your tank and do find a leak, here’s a video showing how easy it is to reseal an all-glass tank. This tank is my native freshwater fish project tank, the construction and design of which will feature here on Parlour Oceans. Ethan, founder and coauthor of this blog, will be featuring his saltwater DIY cephalopod system, as well!
Parlour Oceans 
I would also add that copper is a substance known to stay within the silicon lining all glass aquariums and even in trace amounts can wreak havoc on any systems with any kinds of invertebrates.
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Yes, indeed! This is a common problem if a freshwater tank is being used for saltwater. A frequently seen freshwater fish ailment is infections by the protozoan Ichthyophthirius (or Ich for short). Copper compounds are an effective treatment to eradicate freshwater Ich. Copper is deadly to corals and other saltwater inverts, so be careful out there!
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