Once or twice a week water changes?

Question
I have a diamond shaped 30 gallon fishtank with 2 honey red dwarf gouramies, 2 thick lipped gouramies, 2 x kribensis, 1 red dwarf, 3 cories, 1 pleco. I have been doing 30% water change on mondays and 20% water change on Thursdays when I had about 20 fish in my tank I only have 11 know, So is it fine I've instead of doing 2 water changes a week only do 1 40% water change or rather 1 30 water change I was told by you, that the more the better, What is your take on water changes?Is it fine I've I only do between 30-40% water changes once a week?

Answer
Hi Hannes,
You might be able to get by with doing a 30-40% water change once a week. I know with my cories and pleco, they seem to produce alot of waste. My gravel is always really dirty  where they hang out.  Just be careful and test your water to be sure that this amount of water changing is enough for your tank and it doesn't get too polluted. Best of luck!
Karen