On a 20 with 2 fish and 1 anemone you could more than likely get by with a Skilter, low tech, old school filter / protien skimmer. When i first started out in the salt hobby that was about all there was out there. As far as water goes, if you want to keep soft corals and the like, pure is better for sure but for a low tech approach you can use the water from the sink in the kitchen. I used straight tap water for years and years with tetra dechlorinator with decent results, never had a lot of corals at that time but the inverts i did keep did well. In the keys I went to a breeding facility that kept pairs of clowns of all kinds in 29 gal tanks with undergravel filters covered in crushed coral with a clay pot for them to lay on. They had literlay HUNDREDS of tanks set up like this and all of their water came from the county, and trust me the water in the keys is hevily treated to the point you cant drink it due to taste. So long as your not going to get carried away, a plastic hydrometer will work for salinity checks and I always mixed my water in a bucket with a powerhead thrown in it. 10 minutes later i was pouring it in the tank. clowns are tough, especially the farm raised ones. farm raised may take some time to host the anemone you provide but if you feed near it they will learn to get into it.
Short of it is, low tech can work as long as you dont get carried away with what you put in it. 2 fish and an anemone can be done rather cheap and easy, light would be my point of concern and it sounds like that base is covered.
Matt