I have been using sicce scuba heaters. 50w in my 10s and 20s I bought several on closeout. On new setups I buy aquatops which are the cheapest available to me at the fish farm store. I get the 200w for about $13 300w is $18 I think.
I use them as heating elements run on the inkbird controllers.
I have read a few breeding reports from @Fish Folk where they used bottled water in specimen containers. Maybe able to provide some input on different brands.
I looked at this a while back thinking about using it the same way.
I ended up just using inkbird heater controllers on the "important" tanks and run the rest at the mercy of the heaters thermostats
I bought this rack from BJ's. It filled a space between a 2x4 rack and the wall perfectly. I think a 36" rack would work great if you wanted to do 20L instead
I have used mattenfilter foam as a divider in 40b tanks
Bought full sheets from jehmco to build mattenfilters. With the extra I made a few dividers.
Depends on the fish but 3 20H next to each other would give more control.
I ground up some 1.5mm xtreme today with the grinder and it worked just as well.
Looking into a possible future improvement, upgrading to an automatic grinder with a push button.
One handed operation.
I had a little spoon that my friend gave me a long time ago. He said it came with some medication or powder of some form. Anyway it was the perfect size for 10 gallons. So when I was using safe I would just scoop as many times as I needed to in multiples of 10 gallons at a time.
What type of food are you crushing? I like the pepper mill because I can adjust the size of the grind. With the sera discus granules which are pretty similar to tetra tropical granules in size and shape I try to grind down just enough to be eaten while remaining slow sinking. I think with any food if you grind it or crush it too fine it will just cloud the tank.