After a few months (about 9) of using a few of these stuck on the glass under the overflow of a standard HOB
filter in my Goldfish holding tank (they winter over indoors) I find they serve very well as bio filters. The water in the 30 gal
tank that my 3 big fantails spend the winter in has remained clean, stable and clear. And that's no small deal when you have 3 apple-sized goldpigs in it.
In their holding tank I have 3 of these on the glass under the overflow, stacked, so the flow goes in the top one, and works its way down into the other 2 and out the sides and bottom, via the holes all over the caddies.
The caddies hold 3 nylon/plastic pot scrubbies each, but I've found slightly bigger ones that hold 4. You could also fill them full of bio beads or something like that. Pot scrubbies are cheap and actually have more surface area.
The attached pic is a generic one, they come in all sizes and colors, but the ones I use have big suction cups on the backs. They fit perfectly under the HOB
overflow, or could under any point where you have return water going into the tanks.
Every little bit of bio helps.
People with small tanks, or holding tanks, or main tanks who need some effective biological action will find this idea
The nice thing is once they get some color on them, they blend in well and don't look out of place. Little fish like to swim all over em and nibble on the crud growing on/in them.
I wasn't sure they would work well enough to be a good idea but I think in a 30 gal
tank with 3 GoldPigs (4-6" long) for 9 months with only nylon stocking mesh filters to catch the solids, no chemicals, no water changes, and only the backside covered in algae (looks nice, sucks a bit of nutrient out of the water also) and no gravel, with the water chemistry staying stable with no ammonia, nitrite and only 40ppm of nitrate (some plants would eat some of that) I can call the experiment a good one.
In a small tank of common tropical fish, one basket would probably be all the bio you'd need.
I did use scrubbies from my other tanks biofilter where they were already established to begin with.
I've seen these baskets made to fit into corners too.
I thought I'd pass along this stupidly simple idea along. Seems to be working great so far.