jonnys tank is mature if it weren't he wouldn't be seeing high nitrates would he, he would be seeing high nitrites or high ammonia depending on how far the cycling had got. by over feeding you get high nitrates in a cycled tank because this would make more ammonia, then nitrites and finally nitrates, which you are stuck with. if you are sure you really do keep up with your water changes then the only answer to your problem is your water at its source;ie tap what ever, it must be very high in ammonia and/or nitrite and/or nitrate and your bacteria in your tank/filter are producing these high nitrates. the solution, you check your tap water for NH3 NO2 and NO3 and if they are high you either buy your water (RO or distilled or buy an RO unit) or if your water isn't that high in these toxins you do a lot more water changes and test your water frequently and keep NO3 under 40PPM at the very most