I like the approach of trying a single item at a time and as Roy has mentioned, it is not always obvious, so we may have to take several shots to find the right combo. And what I see in tanks does also relate to what I see in the yard. My situation is totally different than Roy as my yard is a small layer of topsoil spread over the underlying limestone. The limestone makes the water very hard and very alkaline and in the yard it makes it very hard for most plants to grow really well as they are only in about 12 inches of soil and then meet a layer of soft limestone that is locally called "caliche" with solid limestone a few inches below. I have CA to no end and often see chlorosis which first made me think a MG shortage. But it is not true, it is simply that the CA is so high that plants can't use the MG that is all around them. Being here for about ten years off and on, I find that I have to work out how to get each plant to do better. I currently have a rose blooming but the leaves are yellowing and falling off!
This is not a simple game we play, so I have fallen back to just feeling good if it works at all, whether in the tank or in the yard!