Usually you will purchase an air control valve to adjust the strength of the airflow.. some air pumps can come with a control for strength, usually the larger ones with multiple valves. But you can purchase the valves and place them on the airline tubes for only a couple dollars.
A sponge filter such as what Darth recommended.. as for the pump itself it depends on how many lines you need - I generally have 5 tanks (range from 10-30 gallons) and a couple brine shrimp hatcheries going on a single pump, the pumps I use have 4-6 valves.. sometimes I will need to use a T valve to cover all my stuff.. and then the control valve to adjust the amount of air.
Here is basically what to look for -
(I don't use this brand, but it's to show you can get a single or multiple valves.. they can come with up to 8 valves depending upon brand). Just depends on what your needs are. Keep in mind, the more lines you place on a pump, the less the flow will be - I would say a maximum of 2 lines per valve.
- most come in one standard size and should fit all pumps. Always get a little more than what you think you will need.. as you will find uses for it - such as using them for drip lines to add new water into tanks, siphoning fry tanks, etc.
- comes in all sizes and shapes. I always prefer to get ones that are for tanks a little larger than the tank I am using it in.. but that's just my preference.
Air control valves
- place a small piece of tubing on one side, and then the rest on the other side. You can turn it to adjust the flow of air. I also use these for my drip system to control how many drops per second/minute when filling up fry tank.
- good for using multiple lines on a single pump valve.
You can purchase wherever you choose.. I love Doctors Foster and Smith, so used them for an example :)