"The problem with such a low pH is that the nitrogen cycle slows down. Ammonia can build up because of this. In a low pH tank it's not a huge deal because Ammonia (nh3) converts to Ammonium (nh4+) which is far less toxic, but if…"
"Depends a lot on your nitrate levels. If you are registering below 20ppm nitrate you are good to go that long between water changes. If your nitrates are higher, then consider doing them more frequently."