Background:
We bought the plant (Crossandra) from Armstrong nursery 7 months ago and it lived in its original pot for 6.5 months... It originally came with three cuttings, one died from after having a black discoloration creep up the stem and all the leaves drooped and it dried out and died. The other two lived on but weren't showing a whole lot of new growth.

I recently acquired the plant and took it to the nursery where I got a clay pot about double the size of the original pot (8in wide originally) and generic indoor potting soil. I removed all soil from both of the remaining cuttings' roots, cleaned the roots with a little dish soap and water, and trimmed off all the brown mushy roots with sterile scissors (about 20% of the root ends were mushy/brown/rotted). I then trimmed off all leaves that had less than 50% healthy green (30% of all leaves) with sterile scissors to allocate energy towards root growth. I've been snipping off new flower buds as they appear for the same reason. I've also been checking and removing aphids manually for the past few days. In total, I've removed 12 bigger aphids, and 20 smaller aphids.

The taller plant just had part of its stem turn brown and mushy (within the last day) so I carefully removed the infected material and cleaned with a diluted hydrogen peroxide solution. I also poured six cups of water mixed with 3 teaspoons of hydrogen peroxide into the soil. I noticed the stem has a few black bumps below the infected area.

What I've Tried:
Washed roots with soap, trimmed infected off
Repotted in bigger pot with fresh potting soil (clay pot to dry it out faster)
Trimmed unhealthy leaves and all buds
Bought grow light since apartment balcony only gets 2-3 hours of direct sunlight a day
Used diluted hydrogen peroxide on roots and stem.

I spoke with someone from the nursery and they recommended waiting a week and repotting once again but into a drier cactus mix as the plant requires a "loamy" soil type that will be able to drain better. I also purchased root growth hormone to use on the roots when replanting.

My questions
- What do you recommend to do?
- Should I repot now?
- Should I buy a second pot in case one doesn't make it so as not to affect the other if one is weaker?
- Is the pot too big and therefore cannot dry out fast enough?
- Should I try a fungicide?

Saving this plant is extremely important to me, I'm willing to try anything. Thank you in advance!

Crossandras are unusually fussy plants that do not like to be fussed with. You are killing your plant with excessive attempts to help it. The problems started with unnecessarily repotting it into a pot that was too large causing the soil around the roots to stay wet for too long and to suffocate. Root suffocation occurs when the roots are deprived of oxygen. It is not a disease.

Washing away the original soil also removed most of the tiny root hairs further traumatizing the plant. Hydrogen peroxide will not help.

At this point, I am surprised that your Crossandra is still alive after all that it has been through and I don't know how to advise you except in very general terms,

Like most plants, Crossandras do best when kept in their original nursery pots with their roots left undisturbed.

Larger pots cause the soil to stay moist longer and prevent proper drying out.

The more a plant is repotted, the more damage is done each time.

Root suffocation is a result of soil staying too wet and that will cause roots to die due to lack of oxygen. It is not a disease that can be cured nor is it a disease that will spread. Chemical treatments will not help.

You may want to take some cuttings and try to propagate them in a humid environment because I don't see much hope for your existing Crossandra.