What are the tank dimensions (length x width x height)?
Octagonal and hexagonal tanks can look decorative but are impractical. They are hard to work out water volumes due to the irregular shape and this can cause problems if you ever have to treat the fish.
They also limit you to the fish that can be kept in them due to the smaller surface area. Fish swim left to right and don't spend much time going up and down (except during feeding). Length and width are more important to fish because they swim that way.
If you haven't bought the tank yet, and you don't mind having a rectangular aquarium, then perhaps look for a basic rectangle shaped aquarium that is 3 foot long x 18 inches wide x 18 inches high, or something like that. Obviously space and money comes into play and if you don't have room for a rectangular tank then a hexagonal or square aquarium will have to do. But generally the bigger the tank and the more surface area (length x width) it has, the more types of fish you can keep.
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What is the GH (general hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).