You have decided that you want to buy an aquarium. You went to the pet shop and the new prices shocked you. There is a another way to do it for less money. Today's aquariums are made mostly of glass with a few pieces of plastic to hold them together. They really will last for decades with proper care. Anything bigger than ten gallons is too expensive to ship and they are very fragile. It is best to look locally. If you are going on a trip and have the space (read SUV), you can check cities or towns on your route. It is probably too expensive with today's gas prices to just drive all that way to get a tank. You can do a search on eBay. You will see the items on the side. You can narrow your search to 50 or 100 miles from your zip code. It helps you to narrow the search also they will be other items listed in the aquarium categories. Most tanks and or stand are pick up only. Freights costs are very high. I did asked a aquarium stand maker what the shipping and he quote over one hundred dollars for a 300 mile journey. I don't think he was padding the cost. Anyway check on the search option for miles from your zip. You can get an idea of the tanks available in your area. It can vary greatly from week to week.
Things to look at. You will like to know the age of the tank. Has it ever leaked? If it is only a leak it is fixable but it might affect the price you are willing pay for it. You really want to know about chips or cracks. These will often get worse with travel. Bouncing in the back of a vehicles is never good for glass. When you see the tank you want to really look in the corner and the bottom of the tank. It is not uncommon for a tank to have chip out the sides where the tank has been bumped or hit. This can get worse when filled with water. Remember they are very heavy when full. The other question to consider is how cheap in the tank and what is the risk if it breaks. If you are living with one story house and some water spill on the floor, it might not be as important. If you are on the third floor of an apartment building, the neighbor downstairs do not appreciate the extra water. I say that from personal experience when I accidentally broke a twenty gallon tank and the neighbor called the landlord. Fortunately nothing was seriously damaged but a book collection or electrical equipment would have been expensive.
Do not be concerned that there are mineral deposit on the glass. If you bought a new tank, there would be mineral deposit on it in a few weeks anyway. If you must remove them use ammonia or vinegar. Both of these will not be toxic to the fish and do a good job removing the calcium deposits.
If the plastic top or bottom of the tank is broken or cracking. It really too expensive to buy to a replacement part. It is cheaper to just buy another tank. I have seen people use tanks with the plastic destroyed but they were putting it in their fish room in the basement.
Also some people cut a hole in the tank for a filtration system. I personally would never do this. I don't trust a tank with a drain hole. They can be patched with aquarium sealant and a pieces of glass. I have bought tanks like this which have worked fine.
Find out what equipment comes with the tank. Filtration systems, heater, light hood, covers or lids, decorations, gravel, fish, plants, nets, food, and anything else. Anything that you will use helps keep your overall cost down while setting up the tank.
I will mention that there are some odd shaped tanks for sales. The main factor for the number of the fish you can keep is the surface area of the water. The bow front which gives you the illusion of bigger but not the space. The square or rectangular tanks will give more room for the fish for a lower cost. There are hexagonal shaped tanks. When I have asked other they are more likely to develop a leak and have little surface area for three times the price or more. Those six foot cylinder hard to keep the water circulation for proper oxygen levels. The wall hanging tanks that are two inches thick and hold three gallons of water for $300. You get the idea without me listing everything. The size tank that gives you the most space for the money is the 55 gallon tank.
There are also acrylic tanks. They are light weight so move and usually no leaking problem. They are hard to clean without scratching the sides. They can became brittle with age. They tend to be more expensive than same sized glass tank. They are an option to consider.
One of the first things that I bought on eBay was a 30 gallon tank that came with a lid and light and nice solid wood stand. They was not much bidding and I won for $33. I was surprised that was a very reasonable price. It would have cost at least $200 to buy it new. I keep it near my eBay computer to watch the while I am waiting to the postings to load. In some places there is more demand and I have seen aquariums sell for almost new prices. You can also watch the local newspaper for ads. Many people discover that it is more work than they thought it would be. I have kept aquarium for fifteen years. They are fun to watch. The kids love them. You can breeding fish, grow plants and watch the underwater world in your living room. I have written a number of fish related guides. Please feel free to look through them.