KH: 20 (tap) and 25 (both tanks)
GH: Can't get a good reading. I tested 3 separate times, adding over 100 drops each time. Just got darker and darker blue. I then tried with just one drop, and really studied it - it looks greenish - maybe yellow? 2 drops still seems greenish. 3 and 4 start to get blue. Could my water really be that soft? We are on a well, which has very hard water straight from the well. After the softener and iron filter, I would still expect it to be somewhat hard, but these tests are not giving me any information.
Stop adding drops when you see any blue. So based on what you have written your GH is about 3 to 4 degrees. which is soft water. The water softener and iron filter should be producing soft water. The GH test only detects calcium and magnesium. your well water probably had a lot of Ca and Mg. However after leaving the water softener most it will have been removed and replaced by sodium bicarbonate (from the salt that you put into your water softener.
KH: 20 (tap) and 25 (both tanks)
The KH test only detects carbonate ions CO3. Typically in natural water most of the CO3 is from calcium and magnesium carbonate in the water. However your water softener removes that and replaces it with sodium bicarbonate. So your high KH reading indicates your water softener is removing a lot of calcium and magnesium.
for the last few weeks, I've been dosing with Flourish and Excel. The crypts seem to be doing better (no new pinholes). But the stunted growth and yellowing anubias are still a problem. Yesterday, I started dosing with Flourish Potassium as well (yellowing, frayed anubias seemed to match that).
Keep in mind that ones a leaf is damaged it typically will not repair itself. So just look at new growth. If the new growth is noramal you have no nutrient deficiencies. If you have no gowth or very slow growth you probably have a nutrient deficiency.
In most aquariums tap water, fish waist, and your substrate. However with your sand substrate is inert and has few if any nutrients. Your water softener and iron filter and also stripped any nutrients from the water water. In you shrimp tank you probably don't have enough shrimp waist to provide your plants with enough nutrients. It is difficult to tell if your larger tank has enough waist to make a difference. I am going to assume you don't have enough fish waist for your plants.
So basically your fertilizer needs to supply all the nutrients your plants need. Unfortunately most fertilizers don't supply everything plants need. Flourish comprehensive is formulated with the assumption that your water has most of the nutrients your plants need. So most of the nutrients listed on the bottle are present at very low levels that are typically insufficient for good plant growth. Osmocote is a fertilizer designed for potted plants. Many of its ingredients may wash out quickly in an aquairium and are removed with a regular weekly water change. Either fertilizer has enough calcium for good plant growth.
I would recommend using a better aquarium fertilizer such as nilocg.com thrive. it should have enough of most nutrients but like the ones you are currently using dines't have calcium. To insure you have enough calcium, magnesium, and sulfur for your plants, fish and Shrimp I would also use a GH booster. Again nilocg.com sells a good one. Addonly enough to your tank to increase your GH by 2 degrees. That should be enough without making your water overly hard and it also adds potassium sulfate. Continue to use excel. These changes should help.