Which ones do you recommed for a 15-20 gallon tank

I'm planning on getting maybe a 15 gallon or 20 gallon tank, and I just wanted to know which filter do you recommend. I was thinking of getting a - Penn Plax Cascade 80 Power Filter, Tetra Whisper Power Filter 20 or a Marineland Penguin 100B Power Filter. I like the idea of a filter looking sort of like a "waterfall". If I get one of these filters will I need a powerhead or an air pump?? What are those for anyways? And what kind of heater will I need for a freshwater setup?? I'm planning on getting 6 neon tetras, 2 dwarf gourami, 2 otocinlus affinis, and 3 Harlequin Rasbora (and if everything goes well for them I'll add 2-3 more rasboras). THANK YOU EVERYONE IN ADVANCE.

First of all, kudos to planning your tank in advance! Your fish stcking plan is reaonable and should work.

Go with the biggest tank possible - it makes it easier to keep water conditions stable. Hang on power filters are great and easy to clean and maintain. Marineland is a good brand and I have found the Hagen Aquaclear filters to be very good, also. I don't have personal experience with the other filters you mention but most power filters should work pretty well, regardless of brand.

You will not need and air pump or airstone with a power filter. Airpumps are for running in-tank box filters and sponge filters or airstones where extra circulation is needed. You don't need these with the power filter unless you want to run a bubble wand because you like having the look of bubbles in the tank or want a "burping" clam or treasure chest!