Geez... no offence but frankly, in a goldfish tank, undergravels filters are more trouble than they're worth. U see, the problem with goldfishes is that they poo a whole lot and all that poo gets choked up in the undergravel and eventually turns anerobic.

Personally I go bare bottom tanks for goldfish, but u may not like that look. So a very thin layer of large river gravel would be good. Note use LARGE sized gravel 'cos goldfishes can be silly sometimes and choke themselves on gravel that fits in their mouth!

I recommend clip on the side filters or wet/dry filters for a goldfish tank. Depending on where u're from there are many makes of such filters....

When 'recommended' filtration for goldfishes is concerned.... go 3 times the recommended for the tank size would be ideal. (They're the dirtiest messiest fish!). So if the filter is rated for a 30gal.... get 2 of them at least. Basically, the more filtration, the better....

When feeding in concern, the concensus among most fancy goldfish keepers is to ALWAYS SOAK DRY FOOD. Somehow because of their 'fancy breeding', they've become prone to swimbladder problems and soaking their food and feeding green peas (husked), seems to alleviate the problem....

Geez.... I'll be checking this site out more often thanks to Khoobg.

cheers.
kev.