Ok, I’ll try to go through this in order, I’m bound to miss something & answer something twice.
The liquid vitamins for humans are the ones, usually found in the baby section of the drug store. When hith has manifested in the external form the baths are more important. As long as the fish is eating, a dose or two in the food should be plenty to knock back any internal protizoans, especially since you have a limited amount of meds & cash.
When I treat angels, and when I did my oscar when he was younger, I dose 40mg/U.S. gallon. I don’t ever recall seeing the liquid, it’s all in powder form here. Look on the metro bottle, or any paperwork that came with it, to see if there is anything stating the concentration, such as mg/ml or grams/liter. I treat for a minimum of 7 days, often 10 to 14. Metro is much cheaper here, 100 grams is $31. I could make a small fortune shipping metro overseas, if it weren’t for the certainty of incarceration for shipping an unknown powdered white substance.
If all you can afford & have to work with is the 6 gallon & 100ml, go with it. The fresh water & vitamins are more important than the meds with hith, the meds play a bigger part when it is internal & they stop eating. As long as the fish is getting vitamins you & Ian should take some vitamins as well, this helps with any illness. Fresh water will help relieve stress for the oscar, some good chicken soup will help you & Ian feel better.
I’ll give you my usual copy & paste from ms word that I usually post for the internal protozoan manifestation, you may pick out a few things I’m forgetting.
This is what I do for angels, you may want to look into the maximum temp your fish can handle. Metro was originally designed for humans, the closer you can get the tank to 98.6F the better.
Put the angel in a quar tank, I usually use a 10 gallon. Increase the temp over a day or so to 90-94F. Treat daily with 40 mg/gallon metronidazole, with 50% water changes daily. If the fish isn't eating, don't feed for the first 3 days. After 3 days, get some frozen brine shrimp. Take a portion about the size of a few match heads, and sprinkle on some metro. You have to eyeball this one, make the shrimp look kind of like a tiny powdered donut. Once it thaws, mix it in & feed. Sometimes they still don't eat for a few days, but once they start to mouth the food & spit it out, the meds seem to get in their system real good. Continue medicating the water & food for 7-10 days. Don’t forget to increase aeration, as warmer water holds less O2.
Metronidazole is sold as flagyl, het-a-mit, and a few other names I can't recall offhand, used to treat hexamita & hole in the head. They usually suggest 20mg/gallon, this often isn't enough. Metro deteriorates in 8 hours, so you could do twice daily water changes. I've done this on the weekends when I have time.
I've also added epsom salt while using metro, this helps with any constipation issues, cleans out the digestive tract. I start with 1 teaspoon per 10 gallons of water, increasing by 1 teaspoon daily for 2 more days to bring it up to 3 teaspoons per 10 gallons. You will have to adjust how much you use when doing water changes. You may want to give this a try.