It is a good idea to compost any soil-peat or soil-manure mix for several weeks before using as a substrate. During the composting, organic matter that breaks down easily is all broken down, and organic matter more resistant to breakdown remains. I prepare a half bucket of peat soil mix at a time and store it in a covered sweater box (like a shoe box, but bigger) and keep it damp. If composted like this, the mix is much less likely to go seriously anaerobic and kill the roots of aquatic plants. In fact, I have never seen it happen.