While I do not own the Ace 300, it is somewhat similar to my AT Pro. Know this, when it comes to metal detecting you are going to dig lots & lots of trash! Get used to it. I'm not being mean about it but that is just a part of this great hobby. Every single person on here can attest to finding enough trash to make their head spin. But once you find some great coins and relics, that is what keeps you going. You have to be patient and tell yourself you'll dig 10 or perhaps 100 pull tabs before finding a ring, coin or some other nice relic.
If you are using coin mode, have you performed a test on coins before you go out detecting? That will help you out quite a bit. Put a cent, nickel, dime and quarter down on the ground one at a time and run your coil over it and learn what your detector is telling you. Good targets, and not necessarily coins, will ring up a fairly constant VDI number while iron or trashy objects will have numbers jump all over the place.
Hope this helps. I'm sure other members will chime in as well. Bottom line.......be patient and learn your detector.