The Rio Mini 90 Internal Power Filter is available for under $30 new, but there are used options available on Amazon for even less and it’s well worth the price, it’s a great little filter for great value.
It has a great flow, which is adjustable if you need less. But it has some power, so if you keep it on full, be sure to direct it properly.
This little pump is quiet and more durable than any other pump it’s size. It is versatile, strong and ideal for tanks around 5 gallons or larger.