You sure it doesnt have foam or mats? You need some sort of mechanical filtration, otherwise, it simply isnt going to work, unless you are going to backflush every day and dont mind 3/4th of the dirt and particles flowing back to your pond. That way it might work to keep your water fish safe, but it still wont work anywhere near optimal and it wont keep any pond clean unless you put a vortex, sieve or whatever mechanical filter befor it.
Im betting that filter has matalata or similar foam in the top, and that is supposedly cleaned also by backflushing. From what I seen, backflushing simply doesnt cut it to clean mats. The only thing that works is taking them out and doing it by hand or with powerwash. And even that stops working at some point.
Anyway, I dont know what that filter costs, but you can build the same thing for next to nothing yourself from a barrel. Just add filter media that you can buy anywhere (or use scrap media), and you can make it a better filter than that tetra by adding airstones to it. Rather than backflushing, add a bottom drain, thats all you need. See the DIY section of this forum.
If you want to buy something ready made, I wont stop you, but you are going to want something bigger than that if you have koi or many goldies. Judging by volume of biomedia, Id say that filter is barely enough for a large aquarium.
