Charles H. Spurgeon on Psalms 105:29

He turned their waters into blood, and slew their fish. So that the plague was not a mere colouring of the water with red earth, as some suppose, but the river was offensive and fatal to the fish. The beloved Nile and other streams were all equally tainted and ensanguined. Their commonest mercy became their greatest curse. Water is one of the greatest blessings, and the more plentiful it is the better, but blood is a hideous sight to look upon, and to see rivers and pools of it is frightful indeed. Fish in Egypt furnished a large part of the food supply, and it was no small affliction to see them floating dead and white upon a stream of crimson. The hand of the Lord thus smote them where all classes of the people would become aware of it and suffer from it.