This is exactly the Method that has worked for me the Best out of the couple times I've dealt with it upon arrival of new fish.
Metro is a mild Antibiotic and does not tax the Kidneys no where near as much as something like Kanamycin/ Neomycin so don't worry about overdosing them. Discus keepers use as much 3-4 times the recommended dose with no problems.
Follow this to a T and you should be good to go if it's a typical case of Bloat caused by protozoan or constipation. If it's a resistant strain of internal bacteria that is causing infection which ultimately is Dropsy then you will need something stronger added to the Metro like Kanamycin (Seachem makes Kanaplex).
Metronidazole is still an antibiotic and should be used for a minimum of 7 days and 10 even better to ensure that all that it does affect are completely killed off avoiding future stronger infections from reproducing and infecting the tank again if high stress arrives. Remember, Stress is BIG Factor here and is why lights off, tank covered, Increased aeration and daily water changes are all important to recovery and healing in this process.
Also using something like Nitrofurazone Powder @ 1tspn/ 150g with the Metronidazole will create a Synergistic effect by not allowing secondary infection or further infecting other fish in the tank.
-Increase Tank Temps to 82-83* as this will increase their metabolism to help process any residual food in digestive tract.
-Increase Aeration in tank as Warmer temps lower the dissolved O2 in water.
-Lights off for the week of treatment as it lowers the stress on them and Cover the tank with a Blanket
-Add Epsom salt @ 1-1.5 tbsp./ 10g (not all at once as this will increase stress. Do it over a day, every 6-12 hours if you don't typically use it.) Epsom salt is magnesium sulfate and aids in regulating bowel movement as well as removing any extra water retention. You can use aquarium salt as it optimizes O2/Co2 exchange over the gill membranes and also aids in supplying electrolytes to the fish to help produce their slime coat. But not a ton as it can counter the effect oft he epsom salt.
Day 1: 50% Water Change- Full dose of Metro every 8 hours (i.e. 8a, 4p, 12a)
Day 2: 50% Water Change- Full dose of Metro every 8 hours
Day 3: 50% Water Change- Full dose of Metro every 8 hours (should stop losing Fish from this point forward!) If not, might be bacterial infection causing Dropsy)
Day 4: 50% Water Change- Full dose of Metro every 8 hours (If Improvement, lightly start feeding metro flake today and forward for 7 more days)
Day 5: 50% Water Change- Full dose of Metro every 8 hours
Day 6: 50% Water Change- Full dose of Metro every 8 hours
Day 7: 50% Water Change- Full dose of Metro every 8 hours
