It happens to everyone at one time or another. Your neighbor has an aquarium for sale at a yard sale, or your mother in law gives you that aquarium collecting dust in her attic. Now what? Do you buy all new equipment or use the old out dated stuff? How do you get rid of all that crusted salt buildup on the hood and aquarium? Why doesn't the light strip work? Hopefully I can help you out with some of these questions. Believe me, you don't have 40+ aquariums without learning a trick or two on how to save some money!
When you get a "new" used aquarium, the most important thing to remember is what not to do. Do not clean the aquarium out with soapy water or with Windex! You should simply clean the aquarium out with hot water. If you like, you can mix in some non-iodized salt to the water which you are cleaning with. I have always found plain hot water to work fine. You may want to use a razor blade to scrape off any algae or calcium clusters which you can't remove from the glass with water alone.
You will also need to clean the aquarium hood and possibly light strip. This is an area which frustrates most people. The solution is simple. Just use vinegar. Vinegar is an excellent cleaner and can make an old aquarium hood shine like new. Just be sure not to use the vinegar on the actual glass. If you don't have vinegar, you could also use lemon juice, although vinegar works best. The light strip will need to be tested. If the bulb is black on both ends you will need to replace it. You can buy a plant / daylight bulb from Home Depot for just a few dollars, which is much cheaper than the bulbs at your pet shop. If your bulb is new but the light still doesn't turn on, you will want to replace the starter. The starter is very cheap to replace and only takes a quick minute to do. I'd take the light strip to the pet shop with you and let them show you how.
Another concern will be the equipment. Most air pumps can be salvaged with a little work. If you have a name brand air pump then you simply need to test it. If it is not working, you can buy a repair kit for less than 1/2 the price of a new pump. The repairs can be frustrating, so if the pump is small you may just buy a new pump. If the air pump is an outdated variety and it doesn't work, you may try using superglue to repair a torn diaphragm. This sometimes will add up to 6 months to a worn out pump. But, in the end, you will need to replace the pump anyhow. If your used aquarium comes with a power filter, you will want to remove the impeller for cleaning. The impeller is a small magnet which spins inside the filter unit, creating the water suction. You can rinse the slime and gunk off of the impeller magnet, improving the efficiency of the unit. If the impeller is especially dirty, you may try soaking it in vinegar or lemon juice. This same technique works for cleaning power heads, which come with some undergravel filter units.
Your last concern will be cleaning the gravel. If the aquarium has been successful and comes from a trusted source, I would not clean the gravel at all. It harbors many beneficial bacteria which will help your tank mature quickly. If the aquarium comes from a stranger or from a yard sale, you will want to rinse the gravel out in hot water.
That is it! You should now know enough to set up a used tank successfully!
Another thing when buying a used tank, I recommend resealing the tank if feasible. If you got the tank from who knows where, you should seal it. It doesn't cost that much for a tube of aquarium-safe silicone sealer from a hardware store. You never will know how much trouble a leak can be, and that is a good thing.
Just to amplify from Gerrad's comments, if you get the aquarium from a yard (garage sale here in Australia) ask the owners if they will let you fill the tank before handing over the money, do it on their grass/lawn just to avoid the disappointment of getting your bargain new aquarium home to have it leak, or worse, explode all over your house.
Additionally, if you acquire an older pump or filtration unit, you may be able to fix it using household plumbing materials. I was able to water seal my old filter with plumbers tape/floss. It is non-toxic, and goes around the thread of pipes to keep water from leaking. Many people use PVC to connect their tanks to their filters, and I'm sure use many other plumbing aisle products to their great advantage, and savings.
Often you will get a box of equipment along with your aquarium, as mentioned above, it may work! Don't get frustrated if you don't know what all the parts go to. We have the internet to look up loads of info to help, but also, lay them out and see if you can piece things together. I had no experience with what I call the sump-pump filters. They suck the water into the canister, through the filter then back into the tank. With patience, I put together the pieces of two filters and saved over 100 bucks! Well worth it!
I see that you recommend using superglue to fix a torn air pump diaphragm, but I would think that RTV used for automotive gaskets would work better. I've tried it with great success!
I agree that you can get really great deals on used tanks. I bought a 200 L just last night with stand for $100. However you should take your time, don't rush right out and buy the first one you come across, take your time and shop. There are many great deals out there, if you just look for them.
In reference to resealing fish tanks. Some silicon sealers have a mildew resistant chemical in them. DO NOT use it in fish tanks. The chemical stays in sealer after drying and will kill the fish. Be sure to read label before buying. I recommend GE brand clear silicone only. It adheres better and dries stronger than the cheaper store name brands. It costs more, but without the disappointments.
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