Water

Water in itself is simple

Keep in mind, you may have to consider TDS (total dissolved solids) and PPM (parts per million) depending on where you live. You may also want to find out if you have a water softener system and if your whole house or apartment is hooked up to the water softener system or if one of your faucets is not connected to it.

Now, if you're wondering when to water. The easiest and best answer is when the top of the soil looks and/or feels dry. Don't worry yourself on watering once every 3 to 7 days. It simply isn't like that. The watering needs or your plant are going to depend on the type of plant, the planter used, the soil mix and humidity. Once you start to get a feel for your plant, you'll just want to check the soil and move on. I also never stick my finger into the soil. At most I'll press my finger on the top of the soil and see if it feels wet. I know this doesn't work for everyone.

Tap Water

Straight Tap water may be high in TDS. Mine runs around 450 TDS PPM without a softener system. Keep in mind, this is not considered safe levels for drinking really. This probably won't cause harm to your plants, but it will help to create hard mineral build up on top of your soil and planters.

RO water - Reverse Osmosis

This is a water purification process that uses a semi-permeable membrane to remove impurities and contaminants from water. In reverse osmosis, water is forced under pressure through the membrane, which allows only water molecules to pass through while blocking larger particles such as salts, bacteria, and other contaminants. This is one of the preferred types of water for people that have house plants. I use a portable RO filter as it takes my TDS from about 450 to 10 to 16 TDS. This keeps a lot of mineral buildup from happening my soil and planters. Plus, I have the benefit of knowing I have removed possible harmful contaminates from my water, and I am free to use whatever fertilizer I want without feeling like I need to break down the possible science aspects.

Water desalination

This is the process of removing salts and other minerals from seawater or brackish water to produce fresh, drinkable water. This is typically achieved through methods like reverse osmosis, where seawater is passed through a semi-permeable membrane that filters out the salt and other impurities.

I have my doubts on how drinkable this is, so I do not recommend it. Some people use it on their plants, but this probably is necessary. You have always but an RO filter that has one, but I'd save for if you are setting up a salt water tank as everything has to start at zero before remineralizing and adding the needed salt.

Water Softener

This is more of a warning. Water softener systems end up adding salt to your water. Over time this will harm your plants, and could even kill them. So make sure you know if you are using a faucet hooked up to that system. If you are, from time to time, remove the mineral buildup from the soil and planter, this could help them in the long run.

Again, water overall, is simple. You can use it however you want. Just don't water log your plants.