Maintaining good water quality and clean filters are the most important points aside from sufficient space and sensible stocking levels in a goldfish aquarium (and basically any other aquarium as well).

First of all, you need to make sure that your tank is cycled if you follow this schedule. There's another thread on cycling tanks here: (Link is not visible to guests. Please register to view.)
It's usually a good idea to keep testing the water regularly, every other day or once a week for the first 6 months and you know exactly how fast the nitrates are building up in your system. The amount of nitrates you get every week decides how much water you change. Typically, you should change a minimum of 50% when keeping goldfish EVERY SINGLE WEEK. If your nitrates are a tad higher than normal, 75% should be sufficient for your weekly water changes. Just make sure the new water is largely the same temperature as the water in the tank! It should ideally differ by only one or two degrees.

Below I have added a picture on how to use gravel cleaners. (This is a tricky one ) I recommend to vacuum the gravel with every water change when keeping goldfish since they make such a mess and most of the dirt collects in the substrate and rots, creating more harmful nitrogen in the tank. Every once in a while (or even every time!), some people remove the decorations from the tank to get to the usually hard-to-reach places under pebbles or drift wood.
Same with the filter media, it depends on how dirty it gets how fast, but in internal filters it's usually a good idea to scrub some sludge off of the filter and give the media a quick rinse in a bucket of tank water once every two weeks or month. If it comes to larger systems with external filters, you can clean them every second month or so. Change out only HALF of the filter media at a time to make sure you don't start an unintentional mini-cycle!!

Always use separate buckets, sponges, brushes and towels for maintaining your tank!! You don't want any strong cleaning chemicals to get into the aquarium and burn the fishes' skin and gills.

Don't forget to scrub the covering glasses! Algae usually enjoy growing on the warm, humid and well-lit surface right under the bulbs, and this will reduce the amount of light reaching the plants and fish below. You don't need to scrub the algae off the side and back of the tank though, the goldfish will just enjoy grazing on a nice carpet of green algae anyway, and it makes great holiday foods compared to automatic feeding contraptions and possibly over-feeding neighbors.

Summary:
+ Change water regularly, once a week minimum. And always 50-75%, this will keep the water quality from deteriorating too much in between maintenance.
+ Remember the water conditioner!! You can also add some beneficial bacteria/bio-filter booster every once in a while in a cycled system as well. Especially when you've cleaned the filters or changed the media.
+ As long as you have a properly stocked tank, weekly water changes should suffice to keep the nitrates far below 40ppm. (Properly stocked = one goldfish per 10 gallons/40 liters of water in a tank no smaller than 20 gallons/80 liters.)
+ If your fish start acting up, ALWAYS TEST THE WATER'S pH, ammonia, nitrite and nitrate. Something might have gone wrong somewhere. If it's as simple as high nitrite or nitrate you can perform a large water change and everything should go back to normal.

I hope this will solve some of your problems and answer any questions, even if it's just how a siphon works

(Don't you think it would be a good idea to create an information section too? There are quite a few articles on the forum by now... )

Thank you so much!
I think the method shown on the pictures is a lot healthier than getting a few mouthfuls of tank water... xD I don't know if I said that, but I tend to get that when I'm starting up the external filter after cleaning it

Hope it didn't get too long or anything.. I tried to keep it short so there wouldn't be too much extra info...
Oh, and in case anyone has anything to add, go ahead!

Just wait 'til the goldies go have a bite of the cats' tongues Happened to one of my teachers' cats, needless to say, it lost interest... xD

Thanks
The smaller siphon I have works like the one on the picture, but the bigger one I have to use on the larger tank is one of those wigglers

Heh, slightly fish-flavored maybe? c:

Nice, have fun with that! Mine has a sort of thing inside the cylinder, so you have to move it up and down in the water to get it flowing, that plastic thingy is sort of pushing the water up but it won't let it come back down. It works, but it's a bit of a pain when I'm switching sides in the tank (There's a flat metal "stick" separating the two halves) and the tank has less water than before >>

Thank you Skwishee! Hope you found something useful here

It happens to all of us in the start I guess Glad it helped you

I think I'll just sit back and watch the fireworks on this one

Oh jeez...I'll just let you handle this, shall I?

could be a troll

You can't keep them in such a small container! The minimum stocking level is 20 gallons for the first fish and 10 gallons for each additional fish, ponds typically need to be even bigger to avoid temperature fluctuations over nights and to provide the goldfish with deep enough water to protect them from predators.

If you have 3 goldfish, you should have a 40 gallon tank minimum.

Oh no!!! Did she just die overnight or something?