What filter need a powerfull one

You have a 29 gallon tank? A Penguin biowheel 350 should suffice. How many fish have you got in there? I've got a 29 gallon tank with a goldie and three snails, it has a 350 on it and it does wonderfully, although I am blissfully properly stocked, that didn't use to be the case though Any larger filter on that tank would just look monsterous.

You may find the current with a large filter in a small tank is the lack of a resting spot for your fish. For an overcrowded tank, what you're really after is a ton of bio media. You need to provide an over abundance of surface area for beneficial bacteria to grow. I would lean to suggesting an oversized cansister filter which you can fill with high grade bio media and yet be able to throttle the flow. I typically do not recommend under gravel filters because goldfish love to dig and they can disturb undergravel units.

You may also want to place barriers in the tank to create a calm area for them to sleep. Otherwise, your fish will become stressed and die.

Thanks,
Peter

A PEnguin 350 or an Emperor 400 are both excellent filters for this size tank. Both of these type filters contain plenty of extra area for tons of biomedia. I am fond of the old style Pengin 350s for their baskets are unequaled for media volume, but the newer clamshells found in the 400 and the new 350 will do a good job, also. I line up all the little ceraminc cylindars standing up in rows to pack in the maximum amount.

More media, as stated above, along with more water circulation to pass the excess waste past that media will take care of a lot of your problems.