It is always best to leave at least 4 inches of growing room for surprise arrivals and extra growing room that might be needed. It also leaves a gap in case the inch estemation is inaccurate. While hexagon and other shaped tanks look very appealing, they are not very good for breeding. Making sure that you find a filter large enough for your tanks gallon capacity is very important. If not, ammonia and other chemical excrement will rise to unsafe levels, resulting in possible death of fish if not caught quickly.
Filters also succeed in removing dust and other floating debris.
It is a common belief that aerators give oxygen to the water. In truth the purpose is to aid in the circulation of the water, and preventing varying temperatures throughout the aquarium. Aeration also makes the water release harmful gasses and pick up oxygen as the bubbles rise. The box will tell you exactly how many gallons in which it was made to aerate. Used under License.