Ok I need advice once again. For a 29gal octagon tank. It houses one redtailed shark and one little tetra. Now I know that the redtail can be aggressive and can not. Now what is the fish least likely to have a problem with him and will use the upper part of the tank. Now the water is pretty darn hard so Angel and such are out. Any thoughts on the very best fish to have the least foreseen problems.