Gourami & Guppy Acting Weird—Help!

[lil_bluebird]

We recently "adopted" an established 30gal tank with a handful of female platys and three tiger barbs. We set up the tank, added QuickStart, and watched the fish closely. They all did great for about two weeks. Then we switched the barbs out so we could add some other kinds of fish (barbs are aggressive). We got two male guppies and a gold ram cichlid with an adorable dorsal defect (don't worry, he's perfectly healthy and very happy). The one guppy died the next day (he was acting weird before we put him in, I just didn't really think about it). The cichlid has been super happy and eating well. Then we got a pearl gourami. It's been four days and he still hasn't eaten and spends most of his time at the top of the tank, not moving. He doesn't seem to have swim bladder issues, just can't swim very well for being weak. He's been a little more active, trying to go after food, but literally barely noticeable. And his tail/pectorals are disintegrating.
Plus my remaining male guppy is acting super weird, just swimming up and down really fast and acting like he's totally oblivious to anything else. He's not gasping for air, but his color is kinda faded and he didn't eat this morning. Otherwise we have three danios, two platys, and a tiny BRT shark. He's a bit aggressive, but has never bothered the guppy at all. No one else is harassing him that I know of, except maybe the danios. They're super active and have chased everyone else quite a bit. Wondering if maybe the guppy is being stressed out by their activity.
We weren't able to do a normal fishless cycle since the fish were already there, but I used the same substrate and filter that the tank came with, and added QuickStart. It's been three weeks. My water parameters are fine, except maybe pH being slightly high. Why are most of my fish fine and these two not?

 

[Autumnsky]

What is your ammonia reading?
Nitrite
and Nitrate?

Test, write down numbers.

Doing a water change is usually a good idea even if you think the numbers are in the safe zone and you have an ill fish.

Do a water change 25-30% then test again. If any numbers are near the high range or the NitrIte is more than 0, do another 30% pwc.

Making sure the water is declorinated and also for chloramines if you have them from your water provider, and a product that treats heavy metals if you have a well and you aren't positive what your readings are for your well water. Pwc water should be close in temperature to your tank water.

If the numbers are still near or above any danger zone do it again.

If you are using dip sticks for testing often they are not accurate. And many do not test for ammonia. Ammonia builds after a few weeks and you do not have a properly cycled tank, or it had a mini cycle, meaning there weren't enough good bacteria to process the fish waste and degrading foods.

If you replaced sponges, filter pads, cleaned the substrate or replaced it, those are causes of not having enough BB / beneficial bacteria.

 

[lil_bluebird]

Thanks. I've had the pet store do full liquid analysis multiple times (I don't have my own liquid test kit yet, though I'm planning on getting one ASAP). Ammonia and nitrites have been 0, nitrates 10 or less. I did find out that my pH was far too high due to our well water being very hard. I'm planning to have my parents and in-laws' well water tested to see if we can use water from them for our tank, instead of constantly buying distilled water to cut ours. I've lost several fish since posting, and right now I'm also fighting an ich outbreak. I'll ask about the heavy metals when I go in later today. I'm currently out of distilled water, so I can't do a water change at the moment, but I will ASAP.