Johan,
Change your flight again and fly to Phuket, or arrive in KL and take a cheap flight up there, great freshwater fishing and some GT's and stuff too.
If that isnt possible;
Pack a very light spinning rod and some soft plastics. do a net search for Haruan and go fish in any piece of water you find ! I was just in Rompin and after sailfishing, a friend and I drove along a main road and stopped and fished drains, flooded wood areas, just about anything with water in it and were getting strikes on small plastic frogs. We ended up catching a heap in a small lake near Rompin town.
KL has some excellent Snakehead (Toman) fishing, but it isnt easy, in fact its very hard fishing indeed. I'm not sure if now is the best season for snakehead hunting, I think that is past. Do a quick review of Francois's website in Thailand and get some tips. None of these are in KL itself, but many lakes exist within about 50 kms that have very large Toman in them. Air Kuning has peacock bass, as do many others. I cant for the life of me remember many of the lake names, and many of the lake names are informal too. I know there used to be a series of lakes out at Rawang which contained double figure Toman. I've chased them round the lakes in float tubes in quite a few occasions.
The lakes/old quarries near the airport are also excellent, but for some unknown reason security seems to throw a friend of mine off all the time !!
Lakes such as Kenyir ( A long trek from KL) are good for Sebarau, as are rivers in Taman Negara - catch them at the start of the rains I think.
Unlike Bangkok, you dont have decent guides and lots of lakes available stocked with fish for catch and release, so expect a tough time of it. I'm not resident but have fished there with a mate who is, so it might not be of much use.
