25-30% weekly, I run 2 aquaclear 110's so I alternate cleaning one every 3rd water change never both at the same time and that's just the sponge and tubing as you have to make sure the filter runs adequately as if you take care of your filter it will take care of you, as far as the op losing fish constantly when doing them weekly it was probably cause of over cleaning /not letting the tank mature enough, I might be wrong it wouldn't be the first but you can definitely over do the cleaning as I too had fish dying because i was doing 50-60% changes weekly, filters weekly, 100% gravel vac weekly, glass /decor armaments everything, I was also over dosing prime as I thought you added the amount for the whole tank not just the amount of water going back into the tank (bucket method not a python, as with a python you do treat the whole tank cause you can't "pre treat" the water before it goes into the tank) , I could do one every 2 weeks but my plants start to hate me and look sickly if I don't do them every week/week and a half, as far as parameters goes I could definitely do one every month, I do feed 3 x a day though with one day of fasting but I think I'm going to go with 2 days of fasting as a hungry fish is a happy fish, still trying to get my plants to actually grow without my fish eating them, was told I have to stock the heck out of the tank with plants and that should help. But yes you can definitely kill fish if you over clean the tank, you should never have to do more than 30% pwc unless the tank is stocked with super sensitive fish, example: discus, or if something is majorly wrong then you should do 50%-75% over a long period of time throughout the day should take about 5-6 hours to do a tank flush, never do it in one go.
As far as filters working better when they are gunked up I disagree, yes they do filter slower which in theory filters twice as it passes through the filter sponges at a slower rate but I run my filters on high but they are modified with filter floss which also in theory is kinda the same as a gunked up filter just not dirty and waste filled as it restricts the flow a bit bit, I don't like the idea of a gunked up sponge putting stuff back into my tank, also a gunked up sponge leads to high nitrates.