Craigslist is a great way to find cheap aquariums; especially the large sizes (50-100 gallons). The best part of all: often owners will include additional items that go with the aquarium, such as the stand, gravel, filters, heaters, décor, fish medicine, and food. Sometimes they even include the fish (if wanted). So if looking for large aquariums, craigslist is definitely a strong choice. Just be sure to inspect the tank for any damage before paying the owner.
How to make a 5 gallon nano reef tank | cheap nano reef tank set-up
The 5 gallon tank set is among the cheap aquarium kits available in the market. This kit has all the required equipment that you may need to set up an aquarium, however, you must buy the available in the market to ensure quality.
Product - Aqua Culture Aquarium Starter Kit, 5 gal
Introducing a cheap (around $4) small tank filter called "Discard-A-Filter" (I'd say one of these is good for an aquarium of up to 2 gallons. Use two and you could probably go up to 5 gallons. Note that you need an air pump to operate it, but these can also be quite cheap and you only need one, even to run two of these filters.) The filter is so cheap because the manufacturer expects you to just toss it in the trash every month and buy a new one (which would actually end up costing you quite a bit over the life of your fish). However, and this is not because I'm so eco-conscious (I'm reasonable: I don't go overboard, but do try to limit unneccessary waste when I can), the idea of just chucking a relatively large, sturdily manufactured object just rubs me the wrong way. It doesn't help that I happen to know that the only thing about the filter that stops working is the activated charcoal (and a smaller quantity of zeolite, which you can also, optionally, add), which absorbs harmful chemicals from the water -- it's absorbed everything it could, and has to be replaced with a fresh batch, there's no getting around this one. So, in this instructable I'll show you what you need to do to keep happily and cheaply reusing this filter, as well as improving its function.
Stocking a 3 to 5 Gallon Aquarium | Wistful Blue