There are low dose H2O2 test strips available that allow you to test for low amounts of peroxide. Then, you can modulate the dose to hopefully keep from killing shrimp and other inverts. I won’t dose peroxide without using them….the reactive level of oxidants is so dependent on organic levels in the tank, and those change with time as it is dosed.The only thing I'm aware of that is relatively safe in your DT while still being potentially effective for a treatment is H2O2 dosing. Your zoas will be upset and any shrimp are likely to die, but everything else seems to do OK. Cleared velvet out of my system despite not being able to go fallow (similar reason to you, couldn't catch my firefish). Results are not consistent for ich unfortunately.