I hate to say this but NO, there isn't any easy ways of raising Angelfish fry. If it were easy, EVERYBODY would be doing it successfully. That's just not happening.
The gunk you see is leftover foods, fish wastes and algae type stuff in the water. You also brought over bioballs from the main tank so that could also be the source. It's not really an issue so don;t panic over it. The fish tank is not going to be sterile clean and the fry will adjust.
Part of the problem you may be experiencing IMO
is that you are changing over the tanks instead of gradually changing the water through water changes. Since you are in the experimental stage, you should not be putting buffering into the tank directly. You want to buffer the water you use during water changes. If you aren't doing water changes, you can be taking out a small portion of the tank water every day and buffering it then slowly reintroducing it back into the tank. This way you are buffering only the water you are reintroducing NOT the whole tank. I've repeatedly told you , fish DO NOT LIKE CHANGE so any change should be gradual.
Cloudy water: comes from overfeeding. Next time, you won't be overfeeding from the start so this shouldn't be an issue.
Everything turns to S**t: Well you know the old expression " S**t happens" lol. Get over it and move on.
and the buffer: Alkalinity reduces the acid's ability to acidify so by changing it to CO2
, it would then dissapate through gas exchange ( bubbles) don;t worry about over CO2ing the tank.
I think the time has come for you to stop testing the water so that you don't have to make any decisions. By this I mean, go back to basics. Just make sure that the PH of the water you use for water changes is the same as the PH in the tank you are putting it in to. Let the PH do what it wants as long as it doesn't effect the fish. Don't concern yourself with KH
as these don't harm the fish as much as drastic PH changes do. Since you are moving, put the whole breeding thing on hold until after you move. Unless these fry survive, you should have empty tanks, breeding slates and unused sponge filters and fry food. Give your breeders the slates to spawn on and remove anything that has caused you grief in this experiment (i.e that fake algae covered plant). Get your breeders into bare tanks by themselves and alone. Remove any possible problem from the start so then we can start fresh and see what's happening. As I said before, these spawns aren't going to help us much with answers for future spawns. The experiments have been compromised too many times and just may not be fixable anymore.
Side note: if you still need to use plastic plants as decor, use plants with small leaves so that they are not attractive to the breeders as spawning sites. I'm using artificial bacopa and similar types because they have small, round, closely stacked leaves. NOT a good spawning site but still decorative for the parents to feel more secure.
lastly, are you using strips to test PH or using a liquid test? Liquid tests are the better choice. More reliable than strips. Just make sure you don't use your finger to seal the tube before shaking. Your finger will change the PH. Also, you want to make sure you are holding the reagent bottle straight up and down as this too can alter the reading by not having a similar drop each time if not held up and down.
Okay, this is lastly, Guppies are NOT the most resilient fish and I mentioned before do not like lower Ph water. This could also be why they are not doing so well.
That's it for now.