We use this in the back of our fluval 3 ...
930582
We use this in the back of our fluval 3 gallon tank and it is the only small heater (tried two before this) that has kept the tank above 72 degrees. Having the temperature setting really helps but doesn't overheat or underheat the water.
3340119
The Fluval U Undewater Filter BioMax works as expected with my Fluval U filters.  This is the largest packaging of it that I have seen, so I always buy it here on Amazon because of the price versus getting it at the local fish store.  I have Amazon Prime, so I get Free 2-day shipping and I don't have to pay for shipping on my orders which is very nice.

If Fluval made these in a larger package volume, I would buy that.

Make sure you only remove half your bio media when you clean or make changes in your filter so to make sure you maintain a decent established bacterial content in your tank to maintain a working cycle.  Don't wash or clean under the tap, or the chlorinated water will kill off the good bacteria especially on the bio- media you are keeping in the filter.  Rinse with tank water if necessary.

I definitely recommend these for your Fluval U Underwater Filter.  They are the BioMax media that you use with the filter.  You can add other things as well into the cartridge chamber like carbon, zeo-carbon or ammonia removers if you need it.  Fluval has them available for sale as well, and they are available on Amazon.

<a data-hook="product-link-linked" class="a-link-normal" href="/Fluval-Carbon-100-gram-Nylon-Bags-3-Pack/dp/B0002AQKIO/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">Fluval Carbon, 100-gram Nylon Bags - 3-Pack</a>

<a data-hook="product-link-linked" class="a-link-normal" href="/Fluval-Zeo-Carb-150-Gram-3-Pack-Nylon-Bags/dp/B0002AQKKC/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">Fluval Zeo-Carb, 150 Gram, 3-Pack Nylon Bags</a>

<a data-hook="product-link-linked" class="a-link-normal" href="/Fluval-Ammonia-Remover-180-gram-Nylon-Bags-3-Pack/dp/B0002AQKK2/ref=cm_cr_arp_d_rvw_txt?ie=UTF8">Fluval Ammonia Remover, 180-gram Nylon Bags - 3-Pack</a>

Make sure you are buying the correct BioMax bio media.  These are often mistakenly confused with the larger BioMax Bio Rings which will not work or fit in the Fluval U series underwater filters.
4488846
I maintain a 90 gl reef tank and this is sitting in an Eshopps sump (which is a flimsy piece of crap you should avoid for many reasons).The tank has a substantial bioload with a dozen fish total. I have found this skimmer to be easy to clean and adjust. It is superior in its ability to pull nice, dark consistent skimmate to other units I've worked with like SWC120, AquaC EV120, Bubble Magus NAC3 to name a few on similar sized systems with heavy bioloads. The only complaint so far from the client has been a few times the cup lid started rattling loud enough that he had to adjust it. I've looked at it the following day and can't detect any reason it may have come loose. Other than that, which could probably happen to any of the aforementioned units except the AquaC EV series due to design, I'd recommend this skimmer...just not their sumps.
4479156
This is a great product for temporarily housing fish. I'm afraid it isn't large enough for fish in the long-term, but it works well as a quarantine/transport tank. I use it to transport my fish when changing apartments and it is very convenient for 2-3 days of use.

Side note: Despite the marketing of small tanks like this, the minimum tank size for a betta should be 3 gallons, but preferably 5-10. Even bettas need filters and heaters. There simply isn't enough room for either of those in this tank, not to mention very little space for the fish to move around. If you think your betta is happy, put it in a larger tank and see what happens! You'll be surprised how lively and personable bettas can be.
1911023
this filter came with my tank. It's a fluval 204 and i really like it. it has multiple filtration baskets and places for media. It has a input and output which connect to a valve which connects to the canister. water comes in through the strainer through the hose and into the canister then it goes through foam, carbon, and bio rings. over all this filter works really well. even though i have the older model it's still going strong. Parts are a bit expensive but other that that this is a awesome filter.
4699554
Cheaper than the Fluval alternative, but functions really well. Was able to clear the 20 gallon tank water within a day after tank was set up. I also liked the spray bar function, and 3 different filtering media
1255616
so far, so good. I have an established 20gallon long planted aquarium with tetras. this fit nicely behind the tank, we hung it to save floor space on the dresser my tank is on. pretty easy set up, although it was missing part N (one of the plastic elbows to connect to the output spray bar). We jerry-rigged that by cutting a length of the larger hose to attach to the spray bar and output connection. I put in my old ceramic rings/bio balls from the old HOB filter I was running (an aqua clear) and voila!

Note: you should ditch the carbon filter pad, just use the sponge and floss stuff and Purigen for crystal clear water quality!
2207858
I have a Spec 3 and I found this to be the perfect size for it.  If you are thinking small or mini go with the mini.  The Fluval is glass and I don't have an Acrylic tank so I can comment on whether it scratches those.  I can say that the glass walls of my tank look great after cleaning.

Pro's:
It's small
It floats

Con's:
The in tank half of this device is about 3/4" deep.  If you have driftwood or rocks near the walls you probably won't be able to get this in there to clean.  I knew this before purchasing so no starts removed.
4205647
Perfect for my betta! The water pump pushes the betta food around, and I don't have time to unplug it and wait for the water to settle.

Please just note the size because it is a bit large for my fluval spec 3.

The only reason for the 4 stars and not 5 is the price. This should not be $17.

Mine was shipped well and was in good condition when arrived.
4881324
I used this for my Betta fish while we were gone for 10 days over Christmas.  I set it up a few days before we left and tested it out, all seemed well.  When we came home it seemed like it had dropped the food correctly, and our fish was fine!  We use pellet food so we didn't have any problems with the humidity.  I liked that you could measure the food very precisely for each feeding (we do exactly 3 pellets) and that you could put different things in different slots (we included some sinking algae tablets on some of the days for our pleco).  I only wish that it had more slots, since we usually feed 2x per day but being gone 10 days we had to set it up to feed just once per day or he'd have been without food for a few days.
3261557
I own two of these, and like the description says, it works well for a tank up to 55 gallons. I have one of mine set for a 12 gallon tank - the lower top off limited amount - and one for a 55 gallon - the higher amount.

My only note is that as this is designed for a nano tank, it seems reasonable to assume there won't be a sump. In that case, I find this doesn't work amazingly well in a main tank. You have to be really careful with how you position your pumps such that there is very little wave action to force the float to go up and down. I had to really play with my pumps, a tunze 6025 and a jebao wp25 to make sure there was no pulsing action. Otherwise, up and down the float goes, on and off the pump goes, and your tank overflows, given the time.

It does work reliably and it is so well priced compared to other options out there. It deserves 5 stars - you should just understand it's limitations in a sumpless system.
4851150
I love fluval but this doesn't work well at all on low water movement tanks... If you have pumps moving lots of water it holds pretty well... But not on my planted aquarium.. Killed two fish because it overheated the tank...
579428
Light output is great, especially for plants. Even after following the instructions and allowing the mount "cure" for 24 hours, I found the light inside of the tank a couple of days later. This was used on their very own Fluval Spec tank.
398164
This has worked very well the few times my fish have gotten "ick" or had some sort of issue (like float bladder problems) .  I haven't ever lost a fish after using it.
1075919
This came in real handy when I had to be away from the tank for a month.
Don't have to worry about having to ask people to top off your tank in a meticulous manner.
I have it setup on a nano 8gallon NUVO, Fits just fine, but I cant run it with my protein skimmer, its one or the other.
2041243
The Hw302 does an awesome job, and the price cant be beat. I bought this to help out my Fluval 306. I figured it would be good enough to add just a bit more filtration to my tank. I was pleasantly surprised though, I got way more bang for my buck. Plenty of room for your choice of filter media. It even comes with some media in the box. I used some of it, and added my own. The pump is pretty strong and seemed to prime right away. Definitely a great deal, so I won't waste time typing about how much I love it. I will instead point out things that weren't great, but definitely not deal breakers.

The instructions are horrible!! When they translated the instructions to English they used someone that barely spoke English themselves. Spending 10min. watching YouTube videos on this product fixes that. I cant stress enough how important this can be.
The Tubing is really hard. Routing the lines behind my tank were a pain, especially if your tank is up against a wall. I just kept telling myself, "You saved a ton of money, so shut it".
The media isn't the greatest, but it gets the job done until you can get better stuff. I did use some of it, mixed with media I had laying around.
Even with these small quirks, I do highly recommend this canister. Again, please take the time to research this on YouTube, you will be an expert on it before it even arrives at your door.
4070663
If you have a Fluval G series filter these work very well.  Can't say other media wouldn't work just as well, but these are convenient.  Would probably work in any filter that has a bulk space for Biological filter media.