Are you mixing up TDS and GH test kits? If your water GH is 250ppm there is no way plants can lower it to 50ppm. Especialllywhen your nitrate reading is zero. TDS is an electrical device that measures total dissolved solids. TheGH test however only measures Calcium Magnesium. It is possible to have a high TDS level and yet have soft water. I think you need to clarify this a little. It's probably not a big deal but mixing up test names can cause confusion.
As to king a micro fertilizer you will need:
Iron DTPA Dose 0.1ppm
manganese sulfate, Dose 0.05ppm
Boric acid , Dose 0.02ppm
zinc sulfate Dose 0.02ppm
copper sulfate Dose 0.01ppm
sodium moybdate dose 0.02ppm
Nikel sulfate dose 0.005
Again loud wolf is a good source except for iron and molybdate. They don't carry those. So I linked some source you for those two At the time I purchased them the smallest size I could find was 1 pound. great for a pond but way to much for my 5 gallon shrimp tank.
You can apply these dry to your tank but for my small tank I made a solution. For a solution use distilled water and add distilled vinegar or citric acid so the PH is 6 or less.The reason of the acid is that Iron DTPA decomposes above a PH of 8. Also used distilled water and vinegar as those are pure. If you used regular water mold or fungus could star growing in the solution. As an extra step dedicate one syringe for dosing the micro fertilizer and don't allow it to touch tank water. if you do that it shouldn't go bad when stored at room temperature. The solution will have an amber color.
Due to the small dose for Cu, Mo, and Ni I made a concentrate bottle of it and then take 1ml of that and add it to my 50ml micro solution bottle. I only dose once a week at the water change.
The above doses are based on the ratio of these element in plant tissue. Now some plants may need more or less due to different genetics but I don't believe the variation is a lot. Technically the data I saw indicates 0.001ppm is sufficient for Mo and Ni but for some reason my tank needs more. And as a practical mater your only using the equivalent of a few grains of table salt. So it is hard to get it exactly right When I first made it i ended up with several times the needed amount and the fertilizer did great. However later when I got it close to the target 0.001 it didn't work so well.So you may have to adjust Mo in your tank. A dose of 0.050ppm dosent appear to be toxic.
The copper and zinc levelI use are about 20 to 30 times higher then in Seachem Flourish. It Doesn't bother my breading shrimp. Doubling the dose has also had no apparent effects on the shimp. Many people worry about copper in fertilizers but in reality copper pipes typically provide 90% or more of the copper in a tank. The EPA allows a maximum 1.1ppm in tap water. So when using tap water you may want too have your tap water lab tested first to see what your copper zinc and other nutrient levels are and then make adjustments. I don't use my tap water in my tank but I know it has 0.050ppm of copper in it. A good lab test to use is the tcp-oes test. It will detect about 30 elements down to 0.001ppm. I have use this test company. Note this test doesn't detect nitrogen and has a difficult time with manganese so don't be surpired if yet shows Mn at zero.
Note: Seachem uses iron gluconate with is not PH sensitive but bacteria break it down so it doesn't last long in the tank. If your PH was 8 or higher I would have suggested gluconate dosed at 1/3 the level but 3 times a week and separately from the rest of the micros. But as it is your PH of OK for DTPA.
