Turtle tank filters have to work hard to keep the water clean because aquatic turtles are so messy. To be a little gross about it, turtles poop and pee more than fish do, which makes the water dirtier and can lead to lots of bad bacteria that can make your turtle tank smelly and make your turtles sick.
That's why there are special filters made for turtle tanks. They're made specifically with turtles in mind. The biggest difference is that they have more filtration media, which is the stuff that goes inside the filter to keep the water clean. They also pass more water through the filter every hour than fish tank filters for the same size tanks do.
You don't have to use a special turtle tank filter, however. In fact, most aquatic turtle keepers use filters made for fish tanks. That's perfectly okay as long as you make sure to buy a bigger filter than you would for the same size tank if it had fish in it instead of turtles.
In general, if you're going to use a fish tank filter for a turtle tank, get one rated for two or three times as much water as you actually have in the tank. So if you have 40 gallons of water in the tank, get a fish tank filter that's rated for 80 to 120 gallons. That will give you enough filtration for your messy turtles.
There are three kinds of filtration that a filter provides:
Mechanical filtration means straining out solid stuff like dirt and uneaten food. It's usually provided by a sponge, aquarium filter floss, or other media inside the filter that traps particles by straining them out. If your filter uses bulk filter floss rather than a custom-fit pad or cartridge, then you can also use polyester fiberfill pillow stuffing, which is the same stuff but is less expensive.
I personally like filter floss or fiberfill pillow stuffing better than sponges, but some turtle keepers disagree. The reason I prefer it is because it's less expensive, and I can just throw the old floss away. Cleaning turtle poop out of a filter sponge, on the other hand, is a yucky, messy job.
Biological filtration is provided by friendly bacteria who live and grow on the filter media. A lot of times they use the same media as the mechanical filter (a sponge or filter floss); or you can use Bio Balls, lava rock, ceramic rings, beads, or gravel, or other biological filtration media. These are all things that serve as substrates for friendly bacteria to grow on. These friendly bacteria help keep the tank clean by "eating" the turtle poop, ammonia, and nitrites.
Chemical filtration is usually provided by activated charcoal and ammonia filtering media. A lot of times you don't really need it once your tank is set up and the friendly bacteria are established and start doing their job. But if your tank has odors, then you can use aquarium charcoal in your filter to help remove the things that are making the water smell. If your water has too much ammonia in it, then you can use Ammo Carb or a similar product to quickly remove it.
I've been using SunSun canister filters for years in all my aquatic habitats because they're inexpensive, well-built, and very effective at cleaning the water. There are many other good brands, but SunSun is my favorite brand right now. Other aquarists who know me in real life think I must own stock in SunSun because I talk up their filters so much. But I don't. They're just very good, affordable filters.
I have friends who tell me that Penn Plax Cascade canister filters are also a very good value, but I haven't tried them. They certainly are reasonably priced, at least on Amazon.
There are four basic styles of filters used for fish and turtle tanks:
Internal filters, also known as submersible filters, are popular with beginners using small tanks. They usually attached to the glass inside the tank using suction cups. These filters are okay for small tanks, say 20 or 30 gallons, but they don't have enough filtering capacity for larger ones. If you're planning to start with a small tank and raise your turtles from hatchlings, then a submersible pump is probably a good choice because they're inexpensive and easy to install in a nursery tank. But when your turtles grow and you need a larger tank, you'll also need a larger filter.
One thing you should be aware of is that the smaller the filter you use, the more often you will have to clean it. Cleaning turtle tank filters is no fun, so think about that when deciding on a filter.
Be very careful when installing any electrical device inside the tank. You have to be especially careful when running the wire, that it doesn't run over any sharp surfaces that may damage the insulation.
Canister filters are considered by most keepers to be the best kind of filters for a turtle tank. They usually mount under the tank in the cabinet or stand, so they don't take up space inside the tank. Canister filters provide excellent filtration, but they're also pretty expensive for the larger-capacity ones. I already mentioned that SunSun is one exception to the rule in terms of cost. They're quite inexpensive for their quality.
For smaller turtle tanks, another inexpensive canister filter family that I've used from time are the Zoo Med Turtle Clean series canister filters. They're inexpensive filters that are easy to clean. I think they're an okay choice for beginning turtle keepers with small hatchling tanks. The problem is that as soon as your turtles outgrow their nursery tanks, they'll also outgrow these filters; so you may want to consider planning ahead and buying a filter that also will be big enough for your next turtle tank.
Hang-on-back filters (called "HOB" filters for short) hang on the back of the tank. This kind of filter usually won't work for a turtle tank unless the tank has a filter cutout. This is because the water level in a turtle tank is usually kept several inches below the top of the tank so the turtles can't escape; and because water seeks its own level, most HOB filters won't work if the water level is lower than the filter. The pump won't be able to siphon the water into the filter.
In order to use a HOB filter on a turtle tank, you either need to be using a tank with a filter cutout, or to be using an above-tank basking area that allows you to fill the tank to the top with water. Also, because HOB filters are designed to be used with fish tanks, you need to use one rated for at least or three times the amount of water in your turtle tank. Turtles are messier than fish and produce more waste.
Traditional UGF's worked by drawing water down through the gravel by drawing it up through the vertical tubes using aquarium powerheads. The idea was to use the gravel as a medium for friendly bacteria to live in, and then drawing water through it. It seemed like a great idea. The problem was that sometimes, some of the poo and leftovers would get drawn down really deep down into the gravel. That would clog things up, and the decaying poo and leftovers would increase the levels of bad bacteria, ammonia and nitrites.
Then someone said, "Hey! Let's try pumping the water the other way," meaning pumping it up through the gravel. That arrangement is known as a "reverse-flow under-gravel filter." The idea of doing it that way is that the nasty stuff doesn't settle down into the gravel. Many people still use reverse-flow UGF's and think that they're great.
Personally, I'm not so crazy about them. I used to use reverse-flow under-gravel filters, but I don't anymore. The problem is that as time goes on, the filters still get clogged. The pumps draw in tiny bits of floating debris like uneaten food, and pump it under the UGF where it can't be cleaned, eventually clogging it up. So I don't use them anymore. But other keepers I know, including one who has a PhD in biology, swear by them.
Whatever type of filter you choose, you should leave it running all the time. When the filter is not running, the "good" aerobic bacteria in the media will die and the "bad" anaerobic bacteria will multiply because of the absence of oxygenated water passing through the filter. When you turn the filter back on, all the anaerobic bacteria in the filter and all their poisonous waste products will be pumped into your turtle's water. Your turtles could become ill or die as a result.
It's okay to turn the filter off for a few minutes to clean it or rearrange the tubing when necessary, but not much longer than that. If it's not running for an hour or more (for example, if you have a power failure), then change the media and fill it with clean water before turning it back on.
"Filter media" means the stuff inside the filter that's doing the filtering. Turtle tank filters usually use the same aquarium filter media as is used in fisk tanks.
Common filter media include special sponges, polyfill or fiberfill floss, activated charcoal, Bio Balls, ceramic rings, and lava rock. Some filtration media, like filter floss and charcoal, get thrown out when they're no good any more. You can't clean filter floss, and the charcoal will be filled up with whatever it was filtering out, so it's generally thrown away and replaced.
I like to place certain filter media that's granular or small in size, like activated charcoal or ceramic rings, in filter media bags to make filter cleaning and media changes easier. You should also use filter media bags if you're using peat moss or aquarium peat to lower the pH in the water or crushed coral to raise it. Loose peat moss will muddy the water, and any temporary filter media that you use to correct water chemistry problems should be easy to remove when its no longer needed.
Other media, like sponges, can be washed and re-used several times. Filter sponges usually will last for several cleanings before they need to be replaced, but a lot of people just throw them out and replace them because they can get pretty gross and yucky. Lava rock or ceramic rings, beads, or gravel can be cleaned and re-used until they wear away. Bio Balls can be re-used pretty much forever.
Never use soap or detergent to clean reusable media like sponges! Just rinse the media in water drawn from the tank itself to retain the friendly bacteria in the media. You can also rinse them with warm tap water if they're very dirty, but this will probably wash the friendly bacteria down the drain; so if you do this, try not to clean all the media in the filter at the same time.
If your tank has a substrate like gravel or Fluorite, it will serve as an excellent medium for friendly bacteria, too. But you have to keep it clean with an aquarium vacuum cleaner or else anaerobic (bad) bacteria will also start living in it.
There are some dangerous things about aquarium filters and keeping turtle tanks clean. One is that water and electricity don't mix, so you have to be very careful. Make sure you always observe these rules:
Another dangerous thing about working on turtle tank filters is that they get filled with turtle poop, germs, and other nasty stuff. It's their job, after all, to remove that stuff from the water. But the stuff it removes contain pathogens that can make you ill; so make sure you observe these safety rules when changing or cleaning the media in your turtle tank filter:
Earlier in the page I mentioned that you should try not to change all the filter media in your turtle habitat at the same time. The reason for this is to make sure that you always have enough good bacteria to keep your turtle tank water clean. One good way to do this is to use two (or more) filters and to only clean or change the media in one filter at a time. For example, if you have two filters, you can change or clean the media in one filter this month, and in the other filter next month. This way you're never removing all the friendly bacteria at the same time.
If you use two (or more) filters, the total capacity of all the filters added up should equal at least the amount of water in your tank if you're using filters made for turtle tanks, or two to three times the amount of water if you're using filters made for fish tanks.