You shouldn't do a 75% water change except in an emergency. And this isn't one. You can change 100% of the water in a tank and not loose any fish, it's just you should never have to.
Your tank isn't cycled, no matter how long you ran it empty. The cycle process doesn't start untill there is a source of ammonia in the tank. This may be bottled ammonia, a dead prawn, or in this case, your fish. You may need to do some extra partial water changes to keep the Ammonia and Nitrite levels down while your tank cycles, but unless it's really high, 25-30% changes as needed will keep it under control. Your water tests are OK, but show that the tanks cycle is not working properly yet. An established tank should allways read Zero for Ammonia and Nitrite. If it's higher than that, there is a problem, the filter is still new in this case.
Have a read of the link below, it explains how the cycle works. You are doing a 'cycle with fish', so be carefull or you may loose them to ammonia poisoning. But as it's taken 2 weeks for the ammonia to build up I think you will be OK. The fish must be small in relation to the tank size - that is good.
NO MORE fish untill the ammonia and nitrite levels in your tank are zero. Adding more fish now will just increase the ammonia level. Just do partial water changes if it goes any higher.