What I want is clear information that would help someone buy an aquarium, accessories and fish.
If you know about the subject already and have experience which would give your choice of content that's perfect - alternatively, just pick a sub-section of information and write some useful content.
Some things that I would really appreciate include:
1 - How to decide what size of aquarium to buy (ie. to match the number of fish you want to keep, or if you need to fit a certain space then how to know how many and what type of fish it will accomodate)
2 - How to prepare a new tank to be suitable for fish (water cycling and preparation)
3 - A really nice diagram of how the cycle of water/amonia/ph levels etc. gets balanced in a tank)
4 - How to test aquarium water and what to do about the findings.
5 - Information on what popular fish choices there are and understand those fish (i.e some fish prefer to be in groups of 5 or more etc.)