I'm more convinced about the "mechanical wound during growing" theory... cause the 1st arowana to get DE in my tank was also the one who splashed into a "hanging planter" when hunting big green grasshoppers above the surface (into the plants)... He jumped and felt directly in a big BANG into the hanging planter of Asplenium, on the far left of the tank... and a few days after, he began with DE on the side he was hit... Never been able to confirm this theory with the 2 others... but I heard enough of big-bang-splashes in the evening in summer, when insects find a way to get into the tank from the ouside of the house...