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i have two pangasius (3") and i was wondering if it was safe to add a couple of oscars to the tank . oscars have a reputatuon to be very violent towards other fish but the ones in the petshop(2,1/2") are being cept with pangasius(5inches)and are not retalliating. i tought this would be a lesson for them so when they enter my aquarium they won't harass my catfish.

http://www.planetcatfish.com/forum/viewtopic.php?t=4013

All of the relevant points concerning pangasiid husbandry have been covered in this post. Any attempt to start a discussion along the lines of "I want to keep large fish because they will not grow large in my tank/because I want to/because I think I can" will be deemed trolling.

thanks for the link, it was just that my mother hated the pangasius catfish and wanted somthing colourful like an oscar or chiclid

Oscars are not as agressive as many people think. yes the will push tank mates around but can easily end up on the loosing side of a fight.
if you got a small oscar, it probably would be ok but as the shark cats grow rapidly and they would be a bit ahead of the oscar.
But oscars are messy fish and need powerful filtration and plenty of water changes or there health can suffer.

if you want a bit less hassle try a severum.

just avoid any central american cichlids big or small as they will probably attack and kill your sharks.

messy fish as in managing to turn the water green-yellow or blocking a cheap filter? should they be kept in groups or single?

I would still suggest Tinfoil barbs, I have 2 pangasius and 4 large Tinfoil barbs and it is a very peaceful coexistence, they don't pick on each other at all. You will have no worries about compatibility.

oscars are messy for the folloowing reasons:

chew up and spit out food poluting the tank
generate vast amounts of waste ammonia etc
dig in the substrate and stir up debris.
and yes a cheap filter will get blocked.
high quality external power filters the way to go.