To find HM 05 Defog, go to the Great Marsh and pick up the pokeballs on the floor until you get it. You'll need the Relic Badge and the Fen Badge to use it. (AKA, Gym badge 4 and 5)
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go 2 route 18 and surf. you'll find a ledge. hm05 (waterfall) will be on the ledge.
you do not find dialga in Pokemon pearl
hm05 is flash
You must have HM05 Flash to go through Rock Tunnel. Rock Tunnel is to the east of Curlean City. You can get HM05 by going through diglett's cave(and end up near pewter city) and going south to a buiding to which you will find Pro. Oak's aid. If you own 10 pokemon, he will award you with HM05.
You can find Aron only in Diamond.