Airator or no airator that is the question

After being constantly told that an airator is not neccesairy i ask all of you guys. Also could anyone suggest a setup for a 20 gallon long already has the filter and heater and gravel and a few plants. It is cycled. What fish should I get for FW. Any favorites? I am a begginer and this is my first tank. It has a biowheel filter and an ironic 200 watt heater. I used to have 18 fish all happly living then diasaster struck:shake: 5 fish died over night. I immedently went to the petstore with the fish and water and they found nothing wrong with the water or fish! Next night another 3 went down. Same thing happend. Eventally all but 3 fish survived!!! The surviving fish were 2 zebra dainos and 1 albano cory! :sad I have kept these fish for about 2 months and have concluded that the death's have stopped and it is time for some new fish so please send your suggestions!!!!