how do these things work?

And are they good or bad for goldfish's?

I read here or in a book that they aren't good for goldfish because they like to dig in the sand.

thanks

Undergravel filters work using suction from an air pump. They draw the water down through the gravel, then up through pipes and out again. All the solids get sucked into the gravel and this becomes your primary filter material where all the bacteria live.

I had one once with goldfish and I would never do it again. Goldfish are too messy, too many solids. The gravel sits on a grate and all the broken down matter (like silt) falls through the grate and it's just like having a layer of dirt in your tank. When this layer starts to build up the system doesn't work properly and you have to clean it out. YUK, YUK, YUK - never again. You have to pull the whole tank apart, and it is putrid and filthy.

Undergravel filters may be alright with tropicals but frankly I would NEVER use one again.

undergravel filters are bad, and i do not use them. i never will. this is because when the bacteria gets sucked into the gravel, it forms pockets in the gravel and can sometimes make bad bacteria grow. also, you have to clean out underneath them once in a while and when you do that it makes a big mess because it all floats to the top...why bother with all that when you could just use a power filter instead?

I wont say it is bad for under gravel is one of the most successful filter to breed benefitial bacteria.. it is also an efficient filter and it adds on to the look of the tank and allow planting of plants..

but so efficient that many bad bacteria will grow too.. and it is a lot of work to keep gravel clean.. fish also are at risk of poisoning if gravel are not well cleaned (as trapped toxic is released) as well as choking too..

Foam Filters

Undergravel filters do exactly as describes here - all the muck gets sucked uder the plastic plate that is at the bottom of the tank. The plate holds the gravel up, creating a pocket of space under the gravel that you cannot see. In this pocket, beneficial bacteria break the waste down just as an OVB filter does, using the same nitrogen cycle.

The problem with under the gravel filters is two fold for goldies, particularly. The OVB filters will filter out and remove the large debris and the ammonia and other things are what are used mainly for the cycle. You rinse the large mulm and yuck away. The UGF is faced with having to process ALL the waste, including the large eliminations and such. This increases the work load. Goldffish are very dirty fish. They make a lot of waste. An UGF needs to be really really working well to handle this.

Secondly, an an-aeorbic bacteria can quickly set up shop under the filter plate, and instead of producing nitrites, nitrates and oxygen as by products of the waste breakdown, it produces toxic gasses of various natures. (See "hollow ornaments" thread on this site - same thing) These deadly gases can build up in large quantities under that plastic plate. The filter becomes less efficiant and you take out all the gravel and the plate to clean it and that gas is released. Any fish that gets a snoot full of that is dead. You MUST remove all fish from the tank to clean an UGF! It will release tons of debris that is partially processed and potentially release toxic compounds into the tank water.

An UGF can work, but for a large tank with very small fish load. Tropicals in a nicely planted tank, with a light fish load do fine in many cases with UGF. I simply would never recommend one for goldies.

I don't like them. It makes it very hard to clean the tank. And very hard to do a proper gravel vacum.

I suggest you check out the article on undergravel filters on the Aquatics Unlimited site. Just type out "undergravel filters" and your search engine will take you there. After reading the article you will be able to make your own informed decision.
I would certainly not use one.