ok..
Filters.
Airpumps.
UV sterilizer.

what else would improve goldfish tank and water quality?

what about any of these:
- Ion Exchangers
- UV Sterilisers
- Ozonisators
- Oxydators
????

Water changes, water changes, and more water changes.

No filter, UV sterilizer, air pump or anything else will improve or maintain a goldfish tank better than frequent proper water changes.

I agree with the water changes very often. I think size does matter too, I just went from a 55gal tank to a 125gal and I am finding an incredible increase in water quality stability and fish health and growth. I still change 50% once a week and constantly monitor water conditions but have noticed the nitrate level rises much slower.

i have a 14g with 2 small goldfish and my water parameters are fine.

Just wanted something to get it even better

Which is not to say those things are discouraged. It's just not practical to change your tank water 10 times per hour.

Also, a UV will help prevent diseases but are rather expensive and the bulbs need replacing every year. Still, it's an investment in the health of your fish.

For the filter, go heavy on the media and focus less on the turnover. Besides, a big Eheim is a status symbol.

Air pumps aren't needed if you have enough filtration but the goldfish like to play in the bubbles.

Plants help too. But I've given up on plants long ago. Silly fish thinking everything is food. <_<

the most perfect water quality does not lies with the most expensive equipment..

it is about how you circulate the water cycle and making it as close to nature as possible..

The best option is to live by a running river (or at least a pond with lots of vegetation) where natural water resource is available.. where you can build an outlet leading to the river or pond and an inlet leading from pond or river to a filter before going into your main tank.. this way, you can have unlimited supply of clean water 24hrs a day!!

if no river or pond is available, the next scenario you can simulate is to setup a water plant tank where it is used to clean up all nitrate coming from your main tank before passing clean water back to your main tank.. a good water plant tank need not be tall or big.. it is good enough if it is loooong.. where water goes in from one end, and gets clean up when it reaches the other end where it is ready to go back into the main tank.. Using a long tank is to simulate a river.. having one end slightly tilted, and allow the water to flow from the higher end to the lower end.. where the tank is fully planted, and receiving good supply of natural sunlight or electrical lighting..

Good point, Fi5hkiller. A lot of those waterfalls in koi ponds are done with the same logic behind....
A waterfall of lava rocks, and covered from top to bottom with all kinds of plants, and from there the water goes into the pond.Those plants are not pure aquatic plants, but nonetheless. I have seen a whole wall of waterfall full with ordinary ferns, and it has looked stunning. Not to mention that those ferns were extremely healthy and HUGE. Of course, there is usually a oversized filter that does the main filtering, but plants are such an advantage to fish life - it oxygenates the water, takes out nitrates and other waste that plants use up as fertilizer, and yes, Fishbert, its FOOD!
At the old place where we lived I added to each pond a simple 40 gl rubbermaid, just filled with water lettuce and water hyacinth, and a small powerhead got the water to the rubbermaid, while the water ran back into the pond at the other end.

Of course, when looking at the perfect water quality and conditions, you can add all those knick-knacks (some of them expensive) to a tank. Most of those do not replace the basic needs of a fish - loads of space, good filtration, a balanced diet, regular water changes, and no overstocking. I heard a friend talk about the K.I.S.S. method - Keep it simple, Silly!

Fully agreed on water change. Nothing is better than weekly water change. Just do a 10-20% water change weekly and monitor some inportant water parameter like ammonia nitrite and PH and your fish will live a long time. Do not bother with expensive gadgets like ozonators etc. I would propose to have a resonable size filter and a good pump with some good filter media and you should be able to keep fish healthy.