Given that your powrhead is the correct flow for the gallonage, or it has a adjustable flow rate, you can either purchase one of these:
Sponge filtersOr you can just purchase one of the plain-jane sponges for powerheads from someplace locally or online. The regular sponges are a few dollars cheaper thhan the link above.
Sponges make for one of the best bio-medias out there. They have tons of area for bacters to colonize and are easy to maintain.
Here's a link to one of my favorite filters of all time:
Aqua-clear quickfilter attachmentHere is a pic of what it looks like when it is broken down:
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Note the little basket that sits inside of the sleeve. This is the IDEAL place to put some form of bio-media, chemical media, or crushed oyster shells to combat low KH. The water HAS to go through the bio-media chamber before it can be puled into the powerhead and broadcast back into the tank. That way, every bit of water is passing through the bio/chemical media and is being as efficiently cleaned as possible. The floss sleeve that sits in the cage and over the bio-chamber is actually capable of drawing out particulate matter as small as 1 micron. Perfect for trapping nasties that might come in with a new fish or when treating a sick one. A#1 n my book for powerfilters.....