The reason being that salt water would corrode the metal frame that held the aquarium together. Aquariums drastically changed in the 1960's with the invention of silicone adhesive. Metal frames became obsolete and more people started to keep salt water fish and invertebrates. More recently glass tanks have become less frequently used due to the flexibility of acrylic. Guppies mature quickly and usually only grow to be about one and a half to two inches long. There small bodies and feathery fan like tails add a lot of interest to the tank. They are just fun to watch. As with any type of aquarium, there are three basic components to caring for the fish. Diet is very important. As the water in your Goldfish tank ages, it builds up beneficial bacteria that turn harmful chemicals excreted by the fish into harmless ones. However, this will take some time. Start out with only one fish. The nitrogen cycle will not begin until you add the fish, so running an empty tank for several days will not help. Always remove decaying leaves as they tend to drain the nutrients out of the healthy parts of the plant. Dying roots will rot beneath the gravel. Be sure to remove any part of the plant that is dying or appears diseased. Many plants that are seeded into the gravel will reproduce. The new plant growth can be removed and planted in another part of the aquarium. Remember not to leave the light on for more than 10 to 12 hours a day as it may promote algae growth. After adding some snails or crabs, wait another 2 weeks before proceeding. Now at week 6, you will add your first pieces of coral. There are many types of coral used in saltwater coral aquariums. Some of the most common are Button Polyp, Yellow Polyp, Hairy Mushroom Coral and Bullseye Mushroom Coral. Check with your local pet store when purchasing fish to find out what levels are appropriate. They vary amongst fish species. Filter cleaning is extremely important. Filters collect waste excreted by the fish and left in the tank in the form of excessive fool. The filters will eventually fill with debris and be unable to do their job unless they are cleaned.