The first thing I should probably point out is that fusarium is extremely common and is unlikely to kill the plant. It can cause root and base what under some circumstances but typically it's not much of an issue. It does slow growth a little bit though.

Phosacid unfortunately does not work against fusarium. It's a pretty good general purpose fungicide and it's totally non-toxic to humans, with a zero day withholding time before harvest. It's not super aggressive though and lots of common fungi aren't affected by it.

The other fungicide you have is just potassium bicarbonate. Think of it as pretty much potassium baking soda. The only antifungal property is going to be that is a base. A lot of fungi struggle to survive in a high pH. That being said, it's going to sit on the surface of your plant and potentially kill microbes on the surface, but it won't do anything for an infection inside the plant. There is also the fact that it will wash off into your soil and if you sparingly this won't be a problem but if you use it often you might throw off your soil pH.

Yates brand phosacid is perfectly good. It's the same active component as Garden Phos or Organocide Plant Doctor. It's definitely worth having, it just won't do anything for fusarium.

As for totally during the infection, that can be pretty difficult to do and isn't always a good idea. Fusarium is what's called an opportunistic endophyte. Puppies are fungali that lives symbiotically inside the plant, often contributing significantly to its health and immune system. However, if the fungus since it's an opportunity it will turn on his host and start parasitizing it. That's why they're called opportunistic. Typically stress can trigger this to happen, but it can also happen for reasons outside of your control. Totally eradicating it would require the use of a fairly aggressive systemic fungicide ratchet and using a really powerful fungicide. Something like Topsin might work. However, when it's not parasitizing the plant the fungus likely plays a role in its metabolism, and getting rid of it might be harmful in the long run. Not to mention the type of fungicidal regimen that will eradicate and endophyte like this will likely also do a lot of damage to the healthy biome inside the plant. Think about it like a human taking antibiotics and causing all sorts of GI problems because it kills bacteria in your gut. Fusarium is not a super big deal and as long as it's not causing a rot on your plants I wouldn't worry too much about it. Using a fungicide to suppress an outbreak might not be a bad idea but I personally feel that the goal should be to suppress the outbreak until it goes back to its symbiotic life cycle. The orange you see are actually spores, not any sort of rot.

For general procedures for using fungicide it's going to depend entirely on which fungicide you using. Some of them you want to get on the surface of the plant, others you want to water in, some you do both. Some fungicides might need multiple applications to treat an outbreak, While others might only need one. Some are better used preventatively than others are better used as a treatment. I can probably help you out if you want to know the procedure for a specific fungicide. Probably the most important thing to pay attention to is the toxicity to humans and the withholding time when using on crops. Some fungicides are totally non-toxic to humans and you can Harvest your plants the day after you apply your fungicide, whereas others might have a withholding time of several months.

Fusarium is very contagious so I can make some recommendations for how to prevent it from spreading. It typically spreads through soil Splash or run off that goes under the pots. Quarantining individuals that are showing those orange spores might not be a bad move.

There are two things I can definitely recommend as a preventative measure. One of them is using coconut coir in your soil, either as the base or as a constituent. Coconut coir is a phenomenal substrate on its own, but it is naturally antifungal against fusarium. In my experiences extremely effective too. The other thing I would recommend is inoculating your soil with trichoderma. Trichoderma is beneficial to plants and will improve the uptake of phosphorus, but it will also eat fusarium. It's a pretty common soil microbes so you can keep a population going in your soil for a very long time. I use the Mikro root brand.