im about to get a new tank twice the size of the one i have just now and im  gonna buy an under gravel filter,,,can any1 advise me on these filters,,,how do they sort out the amonia and nitrites and nitrates.

There are pros and cons for undergravel filters. UG filters are "old fashion" filters.
It is said that the current build up is not recommended, it also can disturb the growth of plant roots.
But pet stores use them in their tanks.

We do have an UG filter too in the AquaOne , the new "old" tank has one too with a powerhead.
I still vacuum the gravel but I do have the impression that it is not as dirty as it used to be without UG filter.
Towards the UG filter I use another one with activated carbon.

under gravel filters are the worse kind u can get. get get a power head filter (aqua clear is a good one) or a canister (they are more money) or a sump (more money and work. but they are the best) and i know some are going to say sumps are not for fresh water but it can be done.

 what ever u get make sure the filter moves 3 times the tank size per hour

so that means if u have a ten gallons tank your filter needs to move at least 30 gallons per hour. u can never have a filter to big but if u get one to small it could KILL YOUR FISH.

I had a aqua clear 100 on a  10 gallon tank one time it move more than 10 times my tank per hour that was a clean tank

creamkiller: I support Hanna on this one. UG filters are just fine to use in a smaller tank with smaller fish, it would be just enough to take care of their waste. But for a goldfish tank, it NEEDS to be combined with something else.

I think there's an easier way to tell people which strength of filter is needed... because it's usually mentioned on the package what size of tank it's meant for, and in this case, it's usual to have a filter that has at least twice the capacity as the tank it's meant for, so for a 10 gallon tank, you could have a filter that's meant for a 20 gallon, or more. Which I'm saying just for the lulz d:

Sorry creamkiller, need to put this one - your understatement - into place:

We do NOT NEED 2 filters and we do NOT have 2 "not good enough filters"... we decided to have those 2 different types of filters, both excellent quality and high performance. ... even if the UG filter is an "old fashion" type of filtration... IT HELPS US TO KEEP THE GRAVEL CLEANER.
I mentioned this before: when I do the siphoning, I noticed LESS fishpoo and debris vacuuming out of the gravel.
I believe this sounds logical and is easy to understand, also it is PERSONAL PREFERENCE which type of filtration one uses.

I only agrre with one thing you said in your last post on here: that it is good to have 2 filters, in case one breakes there is still the other one working and you are not desperate in case you are not able to buy another one straight away because the shops are closed.

Most fish experts agree that just an undergravel system especially for goldfish is not enough and recommend another type of filtration system like the power filter along with a power head on one of the tubes of the UGF.
Goldfish produce more wastes and what I didn't like about the undergravel system is the parts would get full of algae and were harder to clean and replace, without disturbing the gravel bed and decorations and such AND they just weren't powerful enough for the wastes produced by the goldfish.  And the little filter heads and airstones have to be replaced too quite often.
Cream Killer yes, it is important to have enough GPH and even extra when keeping goldfish.
However, the type of filter/filters people wish to have in their tank is a personal preference.
I really think that the best is the canister filter but I do not have any in my tanks.
I guess if I were to get a 75 gallon tank someday, which I am thinking strongly of doing, I would invest in a fluval canister filter like Clab has for his tank.  IMO that is the best filtration system one can have for a larger tank for optimal performance.

creamkiller: No, just because you have two filters it doesn't mean that both of them are bad.
I've read, everywhere, that it's great to combine different filtration systems, like mechanical + biological or biological + chemical etc. in different forms, and again, ESPECIALLY FOR GOLDFISH.

For instance, I've got an 83 gallon tank, that's 1.30 meters long, and I am going to use both of my power filters to make the water flow as smoothly as possible. This tank would be 317 liters big, and my filters together will be pumping about 1550 liters of water per hour. In other words, here it's good to have two filters so the water will be spread all around this long abomination d:
And I, personally, think that this should be perfectly enough (Even if I have plans on getting an external canister filter one day too... *cough*)

o buy the way u  ever want to get rid of that tank tell me

MY DAD HAS A 125 GALLON I WANT

A  200 would be nice

dad has a 125 and a 100 right below it it is cool

the 125 is about 4 feet long 2 feet deep and i think around 2 1/2 high

 he only keeps koi some are over 2 1\2 feet he also has a 400 or 500 gallon in his shop in the winter but there is no glass to see the fish u can only look at the, from the top

Nicole: Yeah, I've seen that too! It's a pretty fun idea But in the picture I saw the tubes were really long, so I'm just a little concerned about them fish getting stuck in them somehow :'D I've seen some other, smaller example of this where two pretty big hexagonal tanks were connected by two big tunnels! There were tropical fish in it c: (If my memory serves me...)

Aww yea! Might be tricky to find some nice transparent pipes though, don't you think? o_O At least I haven't seen them, and I've been working at a hardware store! xD

Yeah, that's how I imagined things c:

yeahhh... like some on those pics in "Tank Designs"  wow, sooo cool and luxurious