I remember reading this thread one time. I then went on a reading binge of everything related and to this day I'm still scared to death of CSM+B !! Only dose in MICRO amounts that probably isn't doing anything good or bad, but I feel better saying I dose CSM+B once a week !! LOL
Based on everything I have seen I don't believe CSM+B is toxic. However people have found a few problems with it:
1 It uses FeEDTA, this salt is stable at a PH below 6.5 Above that it separates and the Fe may no longer be available to plants. So if your tank PH is always above 6.5 you need to dose more than needed, possibly several times a week to get good results.
2. The levels of Zn relative to iron is very low. As a result a zinc deficiency can occur. Some places have tap water with iZinc in it. Others may have none or way too much.
3. The level of Cu is also very low relative to iron. As a result a copper deficiency can occur. Most homes have copper pipes and very slow corrosion of copper pipes will put some in your tap water. As a result some homes can have very high levels of copper in the tap water or very little. Newer homes with plastic water pipes. Water fromm plastic pipes may have zero copper it in resulting in a copper deficiency.
4. It doesn't have nickel in it. But this is the least used plant nutrient. So tap water typically has enough.
Based on the above you can compensate by dosing more often which may cause B, Mn, and Mo to be too high in the aquarium. All these issues can cause poor plant growth in some tanks while others are doing well. All due to tap water differences.
As a result of some people having issues with CSM+B some people are now making there own micros (including myself) Some of the key differences are:
1 Most are using FeDTPA which is more resistant to the effects of PH and it should be good up to a PH of 7.5. Maybe up to 8. The useful PH range of FeDTPA is a much better match for tanks with CO2 injection.
2. Mn, Zn, Cu, Ni sulfate salts are using instead of EDTA salts. For B boric acid is used and for Mo Sodium molybdate is used. When making a dosing bottle all the ingredients are dissolved in water with a PH of less than 6. Vinegar or citric acid can be used to make the solution water acidic.
3. a higher Zn and sometimes Cu level is used to creat a better balance of nutrients. My first basic micro recipe was Fe0.1ppm, Mn 0.05ppm, B and Zn at 0.02ppm, Cu 0.01. Mo and Ni at 0.001ppm. In my tank this initially worked better than any store purchased fertilizer in my RO water tank with only one dose a week. The only issue that is showing up is that in ICP-OES lab test 3 people including myself have found Mn consumption was much higher than expected. If the cause of this is biological in nature Mn deficiency may be very common. To compensate for the high level of Mn is have to dose it about 3 times a week but I still have not determined how much total I need per week.
So overall the best fertilizers are the ones that are customized to take into account what is in your tap water. Or in my case what is not in my RO water. It is possible to make a microfertilzier that can be added once a week in EI tanks without overdosing.