Hi, and welcome to Tropical Fish Keeping forum.
Now down to business. You have some problems with your mix of fish, and you are only noticing the beginning of these; believe me, it can get much worse.
Serpae Tetra are not "community fish" in small tanks. We have fish profiles here, second tab from the left in the blue bar at the top, or click on the shaded fish name in posts. Please have a read of the profile for this fish and you will understand why you should remove them from this 30g. The gourami and angels are or will be direct targets, and so may the other fish. Serpae need to be in large groups (8+) and in much larger tanks unless they are alone in the 30g.
Angelfish are too large at maturity for a 30g. They are shoaling fish that should be in groups, minimum 4-5 in a 55g tank works (unless you have a breeding pair, but in their own tank). Please read the profile on this fish too.
Corys and all tetras are shoaling fish, needing a group of 5+ (corys) or 6+ (tetra, except Serpae as mentioned earlier). You need to decide on which fish will go back to settle the community.
Now to the water hardness and pH. Softening hard water and lowering the pH can only be managed naturally; do not use chemicals and stuff sold for lowering pH, they will not work with your hardness. The KH (carbonate hardness) of water acts as a pH buffer to resist changes, so the only way to effectively and safely lower the pH is with soft water. This can be done in one of two ways.
Peat extract in the filter. Peat leeches tannins into the water that will acidify it, and at the same time colour it brownish-yellow. The harder the initial water, the more peat and the quicker is becomes exhausted. In you situation I do not recommend it long-term.
Using RO (reverse osmosis) or rainwater added to tap water. If rainwater in your area is free of harmful pollutants, this is an option. A RO unit is better long-term; more expensive initially, but over time cheaper than buying bottled water or something. RO basically removes everything from the water, creating "pure" water that can then be mixed with some tap water to create aquarium water that is suitable in hardness and pH for the fish you wish to maintain.
A last word on fish: selecting fish suited to your tap water is obviously easier than adjusting the water for fish not suited to it. Livebearers do very well in basic (pH above 7) harder water. And there are other fish among the Cyrprinids and Characins that will also be suitable. We can go into all this more later. The above will help get you started.