I love these little sponge filters! They're cheap and effective, safe for fry, and better hosts for bacterial colonies than the more popular power filters. I'm of the firm opinion that the only reason they're not more commonly used is that there's no money to be made in selling them and so they're not widely advertised as an option.

To be fair, they do have two disadvantages: Noise and looks. It requires an air pump to function, and the vibration of an air pump, though quiet, can be irritating to some people. And they're ugly. I have not found the latter to be a problem as they're easy enough to hide behind decorations or plants. The humming of the pump bothers my husband, while I hardly notice it.

The pump does take a little longer to clear the water, and doesn't process large debris very well, but it is effective in keeping the water looking good and in good quality. Larger debris can be removed manually. Cleaning is as simple as removing the sponge and squeezing it out a few times in used tank water (never ever ever use chlorinated water to clean it, as you will kill your entire colony of bacteria and cause dangerous ammonia spikes in your tank). In two years, I've never had to replace the sponge but, when I do, they're only a couple bucks.

And, there is never any danger of fry or baby shrimp getting caught in the filter. Some people take the sponge itself and place it over the intake for a power filter to get the best of both worlds. If I were in need of stronger filtration (for large fish that create large debris, for instance), I might do the same. For guppies, shrimp, and small tanks, however, these are perfect and, like I said, cheap.

I really can't find anything bad to say about this for the price. It looks exactly like the picture and it works exactly the way I expected it to. This is my first sponge filter and I put it in a 2.5 gallon betta tank. That gives you an idea how compact it is.

Obviously, it isn't pretty but these types of filters are designed in a form follows function sort of way and I actually kind of like the way they look. If you appreciate something that is designed to the exact shape it needs to be in order to do what it needs to do, you might even say they're elegant.

I won't list a lot of pros and cons. The main pro (for me) is this. Bettas are hard to deal with when it comes to filtration. The ordinary sponge filter is good for a betta but I think this sponge filter is even better because the outlet is angled 45 degrees and it swivels so you can direct the output where you want it to go. This is an absolute must for betta tanks. Especially small ones. An ordinary HOB is a complete nightmare with a betta compared to this.

I am driving this filter with a small Tetra Whisper pump. Not sure exactly which one it is but it's one of (if not the) smallest they make. And I still have to use a flow valve. At full pressure it's just too much in that little tank. So just about any pump you buy is probably going to be more than enough for this filter. That's definitely a plus because you can buy an ordinary pump for under ten bucks just about anywhere. So for about 15 bucks you have complete filtration for a small tank.

Obvously, the main con is this probably wouldn't be enough for a larger tank with more fish. But then you'd be able to step up to a bigger sponge filter with more pump (flow) for not that much more. The good thing about this one is definitely it's compact size.

Did read the reviews about the suction cup not sucking after a while. I had the idea to coat it in silicone plumbers grease before I used it. Not sure how well that will protect the rubber of the suction cup but I guess we'll see. That would be a major issue if the suction cup came loose in the middle of the night. Most likely this thing would tend to want to float and my lid on my tank is just not heavy enough to keep it from periscoping up and spraying water out. Hopefully that never happens.

This filter works well. The suction cup holds it in place on the back of my aquarium. All you do is hook it up to an air pump with some airline tubing and a check valve, and voila. I have this thing connected to a Tetra Whisper 40 air pump, which is now also connected to an air stone, and both devices still work fine.

This sponge filter spits out water out of the tube on top. It does create a fair bit of flow, which I am amazed at. What's great about the output tube on top is that it's adjustable--you can move it up and down, and rotate the tube to face whatever direction you want.

A few weeks later: I think I paid a little over $6 bucks for this prefilter. It's a great filter, but I just discovered I could've gotten it for HALF as much money from another website. What a rip! Makes me MAD. For all you who haven't bought this yet, save yourselves some hard-earned dough and buy it from[...] Great site, they even take PayPal.

UPDATE:
So yes, sponge filters are stupidly simple. The suction power from the sponges inward is very weak, so it won't tend to hold onto large pieces of plants and things. Don't be afraid, it definitely is working. It's super safe for anything living on or around it. But it definitely does suck very tiny particles toward the sponges. The filter works great. Exactly what I was hoping for. For the record, I'm using it with a cheaper 15 gallon regulatable air pump set to minimum air flow - and it seems to work fine. I couldn't manage any noticeably more or less suction power when I ran my air pump on minimum vs maximum power

I've got the smaller model running on a 20 gallon long with a small bioload and I appreciate how unintrusive it is. I did a lot of planning before getting the item and it seems to be doing fine, but with my setup, it apparently has pretty low suction. This is my first sponge filter, so we'll have to see how functional it ends up being (I have no idea as of yet).

NOTE, I had to shave down with a file a slot for the airline tubing which was too narrow. It's not visible in the item images but the black ring before the top elbow joint has an extended slot to help secure the airline tubing in place and standard tubing didn't fit the hole. Too small of an issue for how happy I am with the item otherwise.

As much as I usually don't prefer sponge filters, this one I can say is pretty great.

I've been using this filter for a 10 gallon quarantine tank, and it's been holding up pretty well. Dual intake sponges are always great, it took a while to get the cycle going with these, but it was a fish-less cycle, so I can't really blame the sponge filter. It does, however, depend on how well the micro sponge material can harbor that beneficial bacteria for the cycle. And it has proven worthy, after the cycle has established, it's been stable ever since.
One flaw, and it's most likely fixable, is that there's a stream of bubbles coming out of the hole where the output pipe goes in, this didn't happen at first, I actually noticed it two months after it's been installed. This does keep the output flow from max flow, but really, it's not a big issue at all.
This is the ideal filter for quarantine tanks, shrimp tanks and other aquariums that might require a low suction from the filter.