Thanks for the info. I've been using distilled without adding any minerals back. Perhaps that's the issue.

 

Lots of possibilities but root rot from waterlogged or infected roots and/or nutrient unavailability are possible culprits. What kind of water are you using? 100% RO or distilled if not supplemented with some minerals and basic nutrient could cause deficiencies (and yellowing) and alkaline, high-pH tap water can "lock out" nutrients even if nutrients are provided (e.g., iron) and cause chlorosis and yellowing.

 

I'm a novice with vivariums, but have been growing houseplants for many years. A few possibilities could be:

• You were indeed too rough with the roots. Aroids tend to get cranky if their roots break, and it may take some time for them to recover.
• There is some rot on the roots, maybe from before you even got the plants. Were the plant roots healthy when you transplanted them (thick and white to pale tan)?
• The plants have a pest affecting them. Mites would be the least visible problem, and may require a hand lens to see. Some mites cause leaves to curl from the edges inward (usually starting from the base), thrips or aphids would cause new leaves to be distorted.
• The substrate has too much/ too little of various nutrients which are affecting water uptake. Since Syngoniums seem to grow in low pH soil [link] that shouldn't be a problem, at least.

If you really are worried, and the plants are large enough, you could cut a small piece of the stem with a few nodes to propagate in water, which tends to work well with Syngoniums.

Anyway, I hope others can chime in with more suggestions!

 

I'm a novice with vivariums, but have been growing houseplants for many years. A few possibilities could be:

• 
  There is some rot on the roots, maybe from before you even got the plants. Were the plant roots healthy when you transplanted them (thick and white to pale tan)?
• The plants have a pest affecting them. Mites would be the least visible problem, and may require a hand lens to see. Some mites cause leaves to curl from the edges inward (usually starting from the base), thrips or aphids would cause new leaves to be distorted.

The roots of the syngoniums were a really dark tan. Definitely far from white or pale tan.

I would probably see thrips or aphids right? If so, I haven't seen any of those. If it's mites, will it affects all the plants in the viv? How would you get rid of them? It's worth mentioning that I treated all of them in a diluted bleach solution before planting them in my viv, was hoping that would kill off anything that was hanging out on them. None of the curled ones have died yet.

 

I don't really have a qualified, academic answer, but I am interested in knowing what the story here is, too, as mysterious and apparently unaccountable yellowing is a problem for me sometimes.

One thing I would personally suggest is getting a moisture meter (which has a prong you stick into the substrate) and perhaps a light meter, which is probably a little more expensive. Someone more informed than me may be able to corroborate whether this is helpful or whether these items aren't really needed.

Also, not that this is relevant to this particular question, but my understanding of mosses is that they aren't watered directly, but lightly misted so that droplets land on their tiny leaves. Not sure if that's what you meant. I'd also like someone with more knowledge to confirm or deny this.