Bi-weekly water changes can be fine, IF your tank is stocked and/or planted accordingly. Yours is not. On top of that, you aren't even doing the bi-weekly changes anymore. What matters is how you care for your tanks now.
The 50% WC every week, was a suggestion that was made because your stocking level is high, and before you claimed everything was fine, you complained about having problems. No one said that it was absolutely necessary to do 50% weekly WCs for every fish tank anyone might have. There is no "one size fits all" rule for every tank, we all know this, which is why we're giving you solutions for YOUR tank, based on the information you've provided us (which changes every time you hear something you don't like). The reason I echoed Subrosa's suggestion for the 50% weekly water changes, is that my tanks are also overstocked, and I leave them purposely so. But I know that I need to adhere to a stricter schedule for my WC's to keep the higher fish population healthy. As I said in one of my previous replies, I haven't had a single issue in my tanks since I started doing that. However, before I started doing it, I had many issues, many very similar to what you claimed to have (before you decided you magically didn't anymore). Water changes are also the least expensive solution for most tank issues and preventative measures, by far. If you aren't willing to increase your WC's, you will need to lower your stocking levels if not also start heavily planting your tanks, or they will develop various problems over time. What you don't seem to understand, is that you're getting advice based on the actual experiences of other fishkeepers, many of whom have been in your situation at some point throughout their years keeping fish (several of whom have far more years in the hobby than you and your dad combined). We're not making these things up out of thin air just to harp on you about your tanks.
Regardless of how many tanks your dad has had for how long without a single water change, yet no fish deaths (not that I buy that any more than your other excuses). Two things are true of any tank, even your dad's. NitrAtes can only be removed by water changes, and the total dissolved solids (TDS) can only be removed by water changes. An excess of either of these WILL cause problems with fish at some point down the line. The problems could be sickness, growth problems, only living to a third of their average life expectency, or any number of other things. This last one is something that most people don't consider. You may have a fish that survives for 4 "whole" years... but on average that species may live to be 12 yrs old when taken care of properly. Also, assuming you are telling the truth about your dad's tanks (and he was telling you the truth) Saying that the fish simply "survived" in a tank, isn't saying much. Humans can live for a while in horrible conditions, but technically be surviving. But how would you like your life to consist of nothing more than surviving? How would you like to live locked in your bathroom, with a non-functional toilet? As long as you were fed, you would likely "survive" for several years, but eventually you're guaranteed to end up with health problems that will shorten your lifespan dramatically, and you certainly wouldn't be living happily during any of it.
