I do 40% a week on all my tanks(they range from 35G down to 5G) but I split it into 2 times a week, 20% on Monday, 20% on Thursday. Internal filters get a clean every 2 weeks, external every 2 months. I also use the vac inbetween water changes if I think the poo is building up a bit.

25%-30% every sunday without fail.

It mostly depends on how much time I have, some weeks i have more than others.

The MINIMUM I do is 2 water changes a week. With one being at least 50% the other being 25% or so.

If I have time, which would be perhaps 20 weeks of the 52 in a year (guessing) I would do 3 50% water changes.

I've never kept discus before but have heard from friends that have and you have to keep the water very clean. I water change every 6 months is WAY to little for any fish, let alone discus.

Some longtime discus keepers and breeder also do 1 or 2 100% water changes daily.

I am using the EI metohd on a 400 litre so i have to change 50% weekly.

On my 'big' tanks its 40% weekly due to the messier fish in there, but on my smaller community tanks its anywhere from 15-30% weekly depending on how I'm feeling.

For my 125G cichlid tank, roughly 30 to 40% per week (sometime over the weekend, but now sometimes stretched to monday due to Baseball all weekend)

For my 55G marine, roughly 10-20% every two weeks as I have a very light stocking and buffer to replace depleted minerals

errr....just saw this was old but I already voted...LOL

I change about 40 gallons every 2 weeks. I have a 125 gallon cichlid tank.
I do this:
1)b/c I only have 18 fish (out of the possible 30-35)
2) They are all juvies and since they are juvies they produce less waste than an adult would

My grandfather and I had the same discussion some months ago. He's kept fish for fifty years, and has always changed 10% of the water every six months, had salt in all his tanks, cycled tanks by repeatedly stocking them with doomed goldfish until a set survived for a week, then switching to the intended stock. He's pretty set in his ways, but he does recognize the better success and lower expense I've had just changing water regularly. He got pretty upset when I showed him the progress with my fishless cycle - he'd brought up a similar idea with the local fish club in the 80's of using ammonia solvent to feed a cycle, and was disappointed when his idea was torn apart by experienced fishkeepers who knew t hat only "natural" ammonia would work.

i do 50% every week on my 100gallon tank, its quite overstocked though.

20% fortnightly on all for of my tanks on a staggered rota

Sean

As I have a heavily planted tank, many toxins are taken out by the plants so I tend to do water changes once a month.
I do about 30-40% at a time.

I used to change around 20% every ten days on my 40G; I've started using Tropica TPN, so I've upped it to around 40%. Can completely recommend the Tropica, BTW.

I do 50% weekly on my 80g oddball tank, a little less than 50% twice weekly on my piranha tank, and 10% weekly on my sw.