As a general rule, don't add anything to tap water to make it suit your fish. Get fish that will live in your tap water, otherwise you're opening up a whole can of trouble.
It's worth remembering, as drrobbyb pointed out, that tap water does have some stuff in it that leaves quite quickly, such as CO2, amongst other things, so you'll also find people using "aged" tap water, which is basically left in a loosely covered bucket for a day or so before using it.
It sounds like you have soft, acidic tap water, which is great for keeping amazonian fish such as cories, so in many ways you're lucky. Your oto's should be perfectly happy in this water as well. They'll probably care more about ammonia and such like than your pH issues. If you want hard water you'll struggle more as you'll need to treat your water to keep a stable tank. Snails tend to prefer harder water conditions, so you may struggle more, but I'm no expert as I'm not a great fan of the apple snails.
Overall, if I were you I'd make the most of what you've got, which is great water for keeping cories, apistos, angels, discus and such like. Many of the most popular aquarium fish. I have the reverse problem, with horribly hard tap water that is pretty much as high in organics and ammonia as they're allowed to have it, which does limit my options slightly with tap water.
It's worth remembering, as drrobbyb pointed out, that tap water does have some stuff in it that leaves quite quickly, such as CO2, amongst other things, so you'll also find people using "aged" tap water, which is basically left in a loosely covered bucket for a day or so before using it.
It sounds like you have soft, acidic tap water, which is great for keeping amazonian fish such as cories, so in many ways you're lucky. Your oto's should be perfectly happy in this water as well. They'll probably care more about ammonia and such like than your pH issues. If you want hard water you'll struggle more as you'll need to treat your water to keep a stable tank. Snails tend to prefer harder water conditions, so you may struggle more, but I'm no expert as I'm not a great fan of the apple snails.
Overall, if I were you I'd make the most of what you've got, which is great water for keeping cories, apistos, angels, discus and such like. Many of the most popular aquarium fish. I have the reverse problem, with horribly hard tap water that is pretty much as high in organics and ammonia as they're allowed to have it, which does limit my options slightly with tap water.