hi there i have two 180 litre juwel vision tanks. one is heavily stocked and is oxygenated by the ventri syatem on one of the fluval internal filters! both my tanks have the cristalprofi e700 external filters in ( which are incredible filters ) one of the tanks i mentioned also has a fluval u3 internal filter which provides the oxygen via the ventri system. my other tank is only oxygenated by the output flow of the external filter. i have positioned it just above water level so it can create oxygen and also is still quiet! also there is fresh water going in every week too! someone i know has a 60 litre tank and has strockedd 2 silver sharks 3 tiger barbs and a fully grown featherfin catfish and they are all still alive after no oxygen what so ever going into that tank for atleast 3 months! so i begun to think to myself do i really need to put an air pump in my second tank and is a high amount of oxygen neccasarry?