1. Never, ever use liquid chemicals to try to adjust pH. It is a guaranteed disaster sooner or later, it will result in wild pH swings in your tank which will go much farther towards killing your fish than a "less than ideal" pH ever will. One of my first tests of any new fish store I go to is I ask about pH (basically play dumb) and see if the employees recommend those chemicals; if they do, I write off that place as a bunch of clueless idiots and never shop there again.
If you have SPECIFIC reasons that you want a pH significantly higher or lower than what your tap water naturally provides, you have two real options to do it safely. The best option is to choose a substrate for the bottom of your tank with buffering capacity for the pH range you want. For acidic pH ranges, ADA AquaSoil is the only product I have heard good reports about. For the alkaline range, there are actually quite a few options depending how alkaline you want; Eco-Complete (which is GREAT for planted tanks, btw) seems to buffer somewhere in the mid-7's, I believe Seachem's Onyx Sand (or Onyx Gravel) is about the same. Then for higher pH's than that you have your various cichlid gravels, aragonite/crushed coral, etc.
2. The other option, if you have a HoB or canister filter, is to add something to your filter to buffer. This is not as effective as replacing your entire substrate, but it can work if you aren't needing to modify the pH too much. For acidic, some sort of peat insert would be needed; for alkaline, crushed coral is probably your best bet.
3. As for salt, it is true that mollies (for sure) and platys & guppies (to a much letter degree) prefer conditions that are more brackish. If you wanted to go the salt route, you would need to be adding marine salt (which contains an amazing variety of salts & minerals), that the pathetic excuse-for-a-product that is often sold in stores called "aquarium salt," which is essentially sodium chloride (table salt). If your tank was an all-livebearer tank, I would actually suggest that you *slowly* work some salt in. However, tetras aren't big fans of salt and plecs even less. You might go with a bit of a compromise; first no salt at all for several water changes until you remove most of the garbage salt out of the tank. Then, if you want to create ever-so-slight brackish conditions, use marine salt at a ratio of 1 teaspoon (not tablespoon!) per 10 gallons. Don't add the salt directly to your tank! Instead, every time you do a water change, add an appropriate amount of salt ONLY FOR THE AMOUNT OF WATER YOU ARE TAKING OUT. So, if you siphon out a 5 gallon bucket worth of old water, then when you put your new water in, put half a teaspoon of marine salt into that new water, dissolve it first, then put the water into your tank. Never dump salt crystals directly into your tank as some fish will mistake the crystals for food and eat one...and that would likely be the end of that fish, their digestive systems can't handle that amount of salt at once.
4. As for breeding, you have 3 of the most prolific breeding species in the aquarium hobby (guppies, platys, and mollies), so it really shouldn't be an issue. However, fish won't breed if they aren't happy. So you need to:
- Get your tank pH (and other parameters) stable. Anything between 6.5 and 8.0 should be fine for live bearers.
- Keep those parameters stable.
- Don't overcrowd your tank.
- Provide plenty of cover (most fish feel much more comfortable when there is adequate places to hide). Plenty of live/plastic plants, including some floating plants, little caves, crevices, whatever.
- Keep them well-fed, with as great a variety in diet as possible. If you currently use a standard flake food, considering buying an all-plant-matter flake food (spirulina flakes), and an all-protein food (brine shrimp flakes), and rotate among them. More variety = healthier fish. You can also supplement with small amounts of blanched (boiled/microwaved & cooled) spinach, zucchini, etc.
- If at all possible, try to include some live food, as this REALLY encourages egg production in female livebearers. If you can't do live, they at least try frozen. Frozen bloodworms are readily available in most pet stores, fish love them, they are very good nutrition, and easy to work with. Break off a little chunk of the frozen worms, put them in a small dish and add a small scoop of aquarium water, give them a few minutes to thaw (even fish don't want to eat something ice cold!), and then dump them in your tank and watch the feeding frenzy. Note that this should be a supplement to your regular feeding; giving some bloodworms maybe once every other day or once every third day is enough for the desired effect.
- As someone else pointed out, with livebearers it is much better if you can keep them at a ratio of 1 male:2 females (or even 1:3). It is less stressful on the girls as there are more of them so the male's attention is spread amongst them.
Good luck to you! Please keep us posted with the results.
