I love the progression of your tank, just read through your entire thread. It's come a long way... I'm trying to figure out why you're having so many issues with parameters in such an established tank? Do you ever vacuum the substrate, do you clean out your filter pad completely when you do your water changes? Beautiful tank!
I also have a 20H, but its going in a slightly different direction than yours, I like to keep it as low maintenance as possible
The new red stem that you got looks really good, is a focus point in your tank, pulls the eye. It also brought back a lot of memories when you posted the AqAdvisor thing, I used it a lot when I was starting out. Here's what it say's about my current stocking
Tank (LxDxH): 24 x 12 x 16 inch (19.9gUS)
Filters: Eheim 2213, Aquaclear 70
Selected species: [Edit Species]
2 x Fancy Goldfish (Carassius auratus)
11 x Harlequin Rasbora (Trigonostigma heteromorpha)
2 x Betta [Female] (Betta splendens)
1 x Peppered Cory (Corydoras paleatus)
1 x Molly (Poecilia sphenops)
5 x Platy (Xiphophorus maculatus)
Your aquarium filtration capacity for above selected species is
Your aquarium stocking level is 208%.
Anyways, keep us updated on the tank. Here's a pic of mine!