You are welcome for the help. I have been doing the cycling help thing now for about a decade. I pride myself on never having failed at this with anybody who followed the directions in the article to which I referred you and which I applied here. The cycling method was designed to be fail-safe. I am just sorry about your nitrite test colors. This was a new one for me. But for most problems there is usually a solution. And I have to confess, I am always happy to see a new fish keeper get their first tank going. This is an amazing hobby in that the possibilities are endless. When I began a lot of very patient folks gave me a lot of help. I am just doing the same thing.
Bear in mind what your new fish have been through. Most were likely born on a fish farm. This may have been on the other side of the world. And some may have been captured from the wild. Consider what life was like for them then. Suddenly they are scooped out of their world, bagged and sgipped, often in stages, until they end up at the store, Conditions there are not ideal as well. Finally, they take their last trip to your tank.
Between the stress and the "shock", they are not exactly in a good frame of mind. They are likely underfed and in parameters which have changed a bunch of time. They are in a small box. So what do you think is their likely response? They will tend to hide. And let's not forget that while your tank is safe from ammonia and nitrite, it will not become fully established for a few months. It will take time for the fish to become settled in their new home.
I would not have chosen white clouds. I keep them. They need cooler water. Sterbais, on the other hand, need warmish water up to the low to mid 80s F. These fish may not do well for you if your other fish are doing so.
The clown pleco, Panaqolus maccus , is basically a wood eater. Here is some info re them from Planetcatfish:
Feeding | A wood-eating fish, the tank should be decorated with several different types of driftwood. These fish really like to have something to chew on. Squash, cucumber, and other vegetables should be readily available to them (yams are a favourite). Supplement two to three times weekly with frozen foods. |
A peaceful fish well suited to the community tank provided it has driftwood. The tank must have good mechanical filtration as these fish produce lots of waste from their steady diet of wood.
https://www.planetcatfish.com/common/species.php?species_id=734
If it is a male, it would like to have a small cave.
Harlequins are an under-rated fish. I have had some in at least one tank for about 16+ years now. They can tolerate temps that both the white clouds prefer and that the sterbai prefer.
The pearls will like the same temps as the sterbai. I hope you got 1m/2fm? 2m will eventually end up fighting over territory more than likely.
The merite is an hernbivore: " Nerite Snails are herbivores and are widely known as one of, if not the most, voracious algae-eaters out of any snail. If there is not enough algae present in the aquarium, their diet can be supplemented with algae wafers or blanched vegetables (such as kale, zucchini, carrots, etc.)." Hikari makes a good algae wafer. The same veggies should be liked by the clown pleco as well
Have you fed the tank yet? Even a scared fish gets hungry. Most of them should come out when food is made available. You need different size/types of food for your fish. The white clouds need tiny food. I will grind up a pinch of flakes with my fingers for mine. I also feed them frozen cyclops and daphnia. Sinking foods are best for the sterbai and they can be fed at lights out. They can find food in the dark as can the pleco.