Your 1gal unfiltered tank is fine IMO/E.....I would use your tap water with the Prime-making twice weekly water changes....1-50% and 1-100% and add a 1gallon dose of the Prime spaced out between the water changes to meet the 48h time line once you have finished the treatment for the fin rot or add live plants like water lettuce, frogbit, duckweed, naja grass, wisteria, hornwort all are big feeders to assist with ammonia/ammonium.
Too many water changes can be stressful on both of you, however, since you are also dealing with fin rot-depending on how bad it is-you may want to make every other day 50% with 100% one time a week.
If he has really delicate fins he may not be able to tolerate the larger tank especially with filtration-some-not all- long fin males can have problems keeping fins intact with too much space and water movement-setting them up for secondary infection.....
Nutrition is really important too-good quality varied diet fed in small frequent meals and if you have any mosquito larva offer a few rinsed for a meal to boost protein intake to promote healing.
If you have a tannin source-either IAL or naturally died and fallen from the tree Oak leaf-this can also help toughen scales/fins, ease stress- as well as the antibacterial/fungal properties to assist with the secondary infection.
Too many water changes can be stressful on both of you, however, since you are also dealing with fin rot-depending on how bad it is-you may want to make every other day 50% with 100% one time a week.
If he has really delicate fins he may not be able to tolerate the larger tank especially with filtration-some-not all- long fin males can have problems keeping fins intact with too much space and water movement-setting them up for secondary infection.....
Nutrition is really important too-good quality varied diet fed in small frequent meals and if you have any mosquito larva offer a few rinsed for a meal to boost protein intake to promote healing.
If you have a tannin source-either IAL or naturally died and fallen from the tree Oak leaf-this can also help toughen scales/fins, ease stress- as well as the antibacterial/fungal properties to assist with the secondary infection.