There could possibly be longterm effects, but there are no clinical studies to support the idea that pet foods can't be fed to fish.
Actually, yes, there are.
One of the main problems with dog food is it is generally far too high in lipid content to be fed to fish on a regular basis. See post #45 in the following link for a clinical study on fatty deposition in fish.
http://www.monsterfishkeepers.com/forums/showthread.php?t=414620&page=5
Fish hatcheries & farms use low quality food as their main goal is quick growth, not longevity. High protein/high fat foods will definitely produce results, but those short term results affect the long term health of warm water tropical fish.