Not positive about the Loaches and the Metro.
The number one thing to do is get the water in perfect condition.
A good number of water changes to start before medication, with the water temperature matched, erring on a touch more warm than cold.
If you haven't been doing very many water changes recently then do some pwc back to back like 20%. And then maybe after a couple hours some additional back to back water changes.
The idea being to get the old water freshened up and remove a bunch of old stuff and gently / more slowly, replace with the newer, less TDS filled water as to not shock the fish.
Identify whether you have the ideal water for the Loaches from your tap. Knowing that can really make a difference for their wellness too. Like not having very hard water.
Low / infrequent water change amounts allow TDS to build and it can really stress the fish over time.
Also topping water off with treated tap water will increase all the stuff in the tank and the minerals and things in the tank do not evaporate but compound. And distilled or RO is recommended for top offs in between water changes. Or just do more frequent regular water changes and not need to refill for evaporation.
What are your tank parameters now?
Ammonia
NitrIte
Nitrate
pH
GH
KH
TDS