I would replace water from your other tank to start.
Long term, if you are going to use tap water, you should probably get something to naturally lower your PH (I know Indian Almond leaves, driftwood, etc. do, but probably not to the degree you need, especially as a quarantine tank). While Gourami prefer a 6-7 PH range, they can adapt to a higher PH if given the time, but they won't be as happy.
The high Nitrates can be combated with plants, and as a quarantine tank, I would do something simple like hornwort (doesn't root, so no substrate is needed. it just floats around and grows, though you can weigh it down and have it look like it's rooted/planted) or the dreaded duckweed (duckweed is a floating plant with roots, but doesn't need substrate since it is really small and won't reach any of the substrate anyway. It is REALLY hard to get rid of so make sure you don't accidentally transfer any to a planted tank unless you have goldfish, but sops up a lot of nitrates).
Long term, if you are going to use tap water, you should probably get something to naturally lower your PH (I know Indian Almond leaves, driftwood, etc. do, but probably not to the degree you need, especially as a quarantine tank). While Gourami prefer a 6-7 PH range, they can adapt to a higher PH if given the time, but they won't be as happy.
The high Nitrates can be combated with plants, and as a quarantine tank, I would do something simple like hornwort (doesn't root, so no substrate is needed. it just floats around and grows, though you can weigh it down and have it look like it's rooted/planted) or the dreaded duckweed (duckweed is a floating plant with roots, but doesn't need substrate since it is really small and won't reach any of the substrate anyway. It is REALLY hard to get rid of so make sure you don't accidentally transfer any to a planted tank unless you have goldfish, but sops up a lot of nitrates).
