Which filter is best?

My first recommendation is the Aqua Clear 200. It is cheaper and quieter with your water level 1/2 inch below the top of your tank. Except that size 200's carbon insert does not fit in the basket very well. It is too long and narrow.

The Marineland Penguin is slightly noisier because of the agitation caused by the bio-wheel. I am still running in my own bio-wheel and I don't see any difference in cycling time from Aqua Clear. But with the new Penguin's two covers, it's much quieter than it used to be. You've got less space for your media cartridges, so less options. But easier to replace media.

If you want even better, Marineland also has the Emperor. I had the 400, but the noise really got on my nerves. So I replaced it with the Penguin twin bio-wheel. Much more relaxing.

I like the bio-wheel. I think someone told you wrong about the Ph and alkalinity it should work in any water that will allow the good bacteria to grow. If the water isn't good enough for a wheel it's likely not good enough for fish or any other kind of filter. They all basically do the same thing but in different ways. Stay away from the undergravel filter, its just a mess. Any other type of filter suited to the size of your aquarium should do the job.

im not a complete expert on filters. i have 90 gallon cichlid tank and use an aquaclear, and i also have an undergravel filter. then i have a 30 gallon community tank and i just have a tetra whisper for that......

i know cascades filter alot of gallons per hour....so maybe look into cascades...

I dont really know but sometimes it also depends on your kind of fish so I would say you should ask a pet shop