Melkor

If you want the highest quality, then cull constantly. Whenever you see mediocre or whatever your cutoff for quality is(look up each color you have, if they have grades, pick the grade you prefer, and cull anything below that), pull them. Generally you only pull the best and restart if you let things get out of hand for a long while. The longer you wait, the more opportunities cull worthy shrimp have to breed and mess up your colony. Put a piece of food in, let them gather, observe and pull.

My api liquid nitrate test went bad after 6 months. I was also thinking something was wrong with my tank. Now I have test aco strips, and plan on getting salifert freshwater liquid nitrate test. Might as well make sure to have secondary test source to check against. Nothing wrong with api mind you, but I've read a few of the testing chemicals are more prone to go bad (nitrate and one or two other I forget) than others, regardless of brand.

Caribsea sunset gold is fairly fine. I picked up 2 50lb bags of caribsea torpedo beach on sale(its an even coarser version of crystal river) for my upcoming 40b planted tank. Its quite coarse, similar to pool filter sand. I have crystal river sand in my 6 gallon betta tank that my crypts and anubia root into just fine. Only downside is the millions of little shrimp poops that accumulate over a week are fairly noticeable lol.

Rhabdocoela

Strips are a good way to get a quick assessment. However liquid and or meters/pen testers are much more precise.

My male betta won't eat the 1mm northfin betta pellets, or similar or bigger pellets. He will gobble up .5mm northfin community pellets, bug bites. I actually really like the new life spectrum betta pellets, they are sub .5mm, just tiny morsels.