Ok....first, I would change your water change volume to 10g. That's approx 30%. On my tanks, I do 25% - 50% weekly and have no issues water quality. Also, since you are feeding flake type food and blood worms, make sure and move the rocks so you can suck any debris stuck under them on your next water change. It may be that blood worms are getting under the rocks and rotting. This can cause an ammonia spike rather quickly if left alone.
This may be a stupid question, but I've seen this done. When you are sucking the water with the siphon, do you just suck the water out, or do you go along the bottom and suck the poo and leftover food particles?
To answer your question on what to add during water changes, I would stop adding anything accept chlorine neutralizer. This should bring your ph down to to a respectable leve. Wait a day after water change, then re-test ph, kh, and gh. Also, set a cup of water from your faucet aside today. Then test tomorrow. Post your results of ph, kh, and gh. Some water is actually high in these areas and don't require any buffering. Better to not add if you don't have to.
I think you are lacking beneficial bacteria because you don't have any type of bio product they can start colonizing on. I am assuming you are using a sponge and crushed coral in your ac70. You may want to remove the crushed coral and put some type of bio media in there. If you do this, don't clean the bio media when cleaning the filter. This is one of the most common reasons for someone running a hob and canister filter on the same tank. hob filters provide great mechanical filtration, while canister filters provide great bio playgrounds for bacteria.
You mentioned the tank has been set up for about 4 months with 1 fish in it. What was that fish? When did you add the brichardi? Were you monitoring your ammonia levels during the 4 months?
I've never used seachem prime, but from what I understand, this is for new tank setups to aid in bacteria forming in your bio media. (I could be wrong). I'm not sure if you should keep using this product or just use a chlorine neutralizer.........someone else who knows more about this could help you more.
As far as your fish flashing, I notice this mainly when there is an abrupt change in water condition (ph, kh, gh).
Since I've never had ammonia spikes, someone else can chime in for more info on how to help reduce current levels.
There are some very smart people on this forum who have alot of knowledge. They should be able to help you more than I can.
If it were my fish, I'd stop feeding them for a few days. I wouldn't add anything else to the tank that could continue to raise the ammonia levels. It won't hurt the fish at all. This will give the tank a chance to start breaking down whatever food/fish waste is in the tank. Also do water changes every day/other day to get rid of that ammonia.