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1. Is micro toxicity a thing? Are there certain nutrients that we can lard on without a problem and others that we have to watch out for?

Certainly, this is well documented for terrestrial plant, but there isn’t much information about this Phenomena regarding Aquatic plant. But, that doesn’t mean it doesn’t exist, we just don’t know how to identify it. I don’t think we should be Larding onto any nutrients, but should be adding them in proper balance as well as in sufficient amount, Boron for example can stop and stunt the plant growth while it also plays an important role for plant growth, Zn can be somewhat forgiving but far as I know it doesn’t serve much benefit if added in excess amount, it could further interact with PO4. This same concept applies to all the other nutrients as well. There are several other nutrients that can be forgiving such as Ca, Mg, S, Cl, Na etc. but they will have some effects on other nutrients once they become excess, Na for example will start melting plants at some point.

2. What is the difference between the various chelators? Do those differences really matter? Does the buildup of chelators cause harm? Is there a problem with using multiple chelators in the same tank?

EDTA, DTPA etc. are commonly used, I have also played with organic Chelators as well, the major differences are how well they could hold on to the particular Nutrient such as Fe, Mn, Zn, Cu etc without any interference from other nutrients (mentioned in Question #1) at certain PH levels. Am not fully convinced if buildup of the Chelators should cause harm to the plants, on the contrary chelator supposed to make the nutrients more available to the plants. If excess Chelator are present in the tank, it should grab any floating nutrients such as Fe, Mn, Zn, Cu etc and prevent it from precipitation depending on the KH/PH. Some of these Chelate adds additional Nitrogen which can be a good or a bad thing, especially for algae. “multiple chelators” I assume you meant EDTA+DTPA+EDDHA and such? I don’t see any issues with this combo, this should further enhance the solution. Since these chelate grab on to several nutrients, even though they become safer, but am not sure how fast plant could extract them from the chelate before they could use them, this is where the problem could arise if the process of extraction becomes slow or hindered. On the contrary free floating Metals are constantly available to the plants such as Non chelated, example: FeSo4, Fe Gluconate, MnSo4, anything with SO4 or Cl far as the trace/Fe goes. While the free floating metals are constantly available, they must be added to the water more often, as they render useless depending on the KH/PH, also in the presence of the higher O2. Some of the metals are more prone to these conditions, while some are hard to precipitate or oxidize, majority of them just fall into the substrate, depending on the acidity level of the Substrate, they could become available to the plants.

3. How do you identify micro toxicities or deficiencies?

Toxicity: plant necrosis (holes, burnt tips etc.) plant chlorosis (color fading, white leaves, yellow leaves etc.) despite dosing unlimited Nutrients. “Toxicities or Deficiencies” above symptoms looks almost the same in both cases. if you have the above issue with higher nutrients, have you tried a lower nutrient? Have you at least tried name brand such as Tropica? Some people suffering from Phytotoxicity but they are under the impression that they have deficiencies going on, someone adding 0.5 ppm Fe and still suffering from Iron Deficiency should look further beside Deficiency at that point. Other issues are that some Fertilizer are not properly balanced to begin with which can cause both Toxicities or Deficiencies, you might have to modify them to be decent. If you have read my “Ultimate Fertilizer Recipe” which is probably outdated now, Here: you will see where I made some tweaks, I focused more on Fe and Mn far as micro goes, in most cases I maintained 1:0.5 ratio for Fe:Mn (still do to present day). I also used Miller at some point and still recommended it to several users over CSM+B because its more balanced in many aspects, also very well mixed unlike CSM+B. you are more likely to see Toxicities or Deficiencies symptoms with CSM+B compared to Miller if they both were dosed at 0.5 Fe proxy weekly. You are less likely to see Toxicities from Non chelated fertilizer if they are dosed in smaller doses more often (even daily), see the answer in #2 question. there are several other indications such as leave twisting, which is also linked to improper Macro dosing beside Micros.

4. Tap water and soil usually have some nutrients. If you have a low-light tank, especially a dirt tank, do you still need to add micros? Does fish food have enough micros to have an effect? Do "shrimp blocks" give off anything useful to plants?

Majority of micros are already present in the tap water, but several of them could be very low or missing, we can only be sure if you have a complete water report, from there you can add only whatever your tap water might be missing. Am not sure if fish food or shrimp blocks would be enough to provide micros, but I don’t think it should be sufficient if the tank is growing plant quite fast. However, in most cases Dirt tank should grow plant just fine, NPK is far more important to the plants than the Traces, not saying trace are not important, but you will likely to see more issues from NPK first if something Is wrong.

5. Is CSM+B a good or bad micro mix? How does it compare to other micro mixes?

See answer #3, in my experience it’s a decent mix, also depends on who adds the correct amount of Boron and which kind, boric acid or borax. some add Borax, which has potential to react with Copper, I personally don’t recommend Borax. There are several other issues with CSM mentioned #3. If you still choose to use CSM+B, I suggest getting the correct data from the seller and see what and how they add the boron, possibly best to get CSM only and later on modify it with Boric acid, Fe, Mn etc. as needed. If not satisfied with CSM+B, then try miller or tenso cocktail, I have used cocktail with great success, even though I cloned it to make the solution as tenso isn’t available in the USA. Try Tropica, this is the best stuff to try when everything else fails, at that point you could rule out several issues.

6. Iron is usually grouped with micros. How much iron do we really need to add? Does the source matter? Is it better to add iron to the water or have it in the substrate?

Not sure what you meant by how much, as you can add whatever amount you want. I usually add 0.1-0.2ppm Fe DTPA weekly, in most cases 0.1 Fe weekly. Here is the tank running with 0.1 Fe weekly
The source of the iron does matter; this would be similar to NO3 vs NH4 example, See answer #2. Iron gluconate is usually preferred source vs DTPA/EDTA etc, Fe+2 vs Fe+3, anything with Fe+2 will be utilized by plants rather quickly vs the other two, but that doesn’t mean the other 2 are not effective. I strongly believe in leaf uptake of iron, the iron in the substrate is mostly in Oxide form but it will slowly become available to the roots.

7. How important are nutrient ratios? Which ones do we really need to worry about? Not just between different micronutrients but also between micros and macros.

My experiment’s indicated that Ratio of all nutrients can play a major role on the plant growth, weather its micro or macro. but there is no way common hobbyist would be able to run such a system. It is almost impossible to have a good working ratio on tank that uses tap water, more luck with the ratio can be seen on RO/DI water setups. I kept the Fe:Mn ratio at 0.1: 0.05 weekly dose for a reason, if you are still seeing issue with plant not turning red, consider looking into other factors, such as Mg, Cu etc., also consider the Lights especially rich in red/blue spectrum. If I shifted away from that Fe:Mn ratio, I found myself adding more Fe/Mn such as 0.2-0.5 ppm etc. when this ratio changed to 0.5:0.1, I also use B:Zn at 2:1 ratio as well, in most cases some of these ratio don’t matter much long as they are not extremely different, for example 2:1 vs 5:1, you might still see good results at 3:1 because it falls between the two. If for example the ratio became something like Mn:Fe 2:1, 3:1 etc. which is the opposite, you might start to see some problems, where plant might appear Fe deficient, soon as you added more Fe in this situation, you should see the positive outcome, because now you have started creating that balance, this rule applies to other nutrients as well. in some cases, if they are present at say 1 ppm Mn and 0.5 ppm Fe, you might see mixed results depending on how plant react to it, some will do fine, while some might suffer.
I have played with a much wider ranges of ratio and am quite confident to say in most cases that these ratios do matter, even though most of your plant might be growing just fine, but plant have more potential when proper ratio is used. In one of the experiment where 0.5 Fe as Proxy vs 0.1 ppm Fe from Tropica was added weekly, plant grew much better with the tropica ratio, it was quite evident that even though 0.5 Fe from miller added all the micros/fe at unlimited levels but it failed to grow several plants (some grew out of control, some stunted, some stop growing completely), while 0.1 Fe from tropica with much lower level grow almost all those plant. This is where Question #3 and #7 plays an important role.
Macros have the same concept, I have played with Ca:Mg at 4:1 and 1:4 ratio, I got better results with 1:4 in most cases with almost 0 algae, but not all the plant grew as expected, they only started to grow once this ratio was shifted to 2:1, 3:1, 4;1 etc. now let’s say if you had 400 ppm Ca and 100 ppm Mg, you should see some serious issues with the plants even though this is within the ratio. This rule also applies to the NPK in a same manner, the best results were achieved at 1:0.1:08 NPK ratio, but it doesn’t mean 1:02:5 or 2:0.5:10 won’t work, but in this case you might find yourself dosing more Iron and in some cases buildup of K over time, in some cases plant not growing as expected, twisting etc. becomes common issue. However, these ratios mostly affect the plants if they are present in the water, in the substrate they work differently and can be at certain ratios.

8. Can dosing or overdosing micros be beneficial or harmful to fish or inverts?

Overdosing of metals are not beneficial to plants, fish or shrimps. However, they are important to all of them, Cu for example is beneficial to shrimps but also deadly at the same time, depending on the 0.006 ppm vs 0.06 or 0.6 ppm.
9. Keeping in mind that plant species have different preferences, and as aquarists we're better off striving for "good enough" than "ideal", in general, what dose should we use for each micronutrient?
Depending on your water parameters and dosing methods, you might want to explore this by making your own Micro’s, I have the link below where you start. But based on my experiments, I obtained best results with Tropica ratio, I also had very good results with Tenso Cocktail Clone. You should see good results at 0.1-0.3 Fe Proxy weekly from any of these.

10. Is there anything else we should know about dosing micronutrients?

I believe I covered most of it in your other questions. But make sure you understand the importance of CO2 and Lights, then NPK and then you could worry about the Micros. If you failed with the first two, it doesn’t matter what ratio you add, you won’t succeed.
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