The fish even in the OP photos is in a dire, advanced state, real bad shape.
Yes, MicrobeLift BSDT can be used in-situ fine, as a long term bath.
I don't find any basis for talking about ponds and anaerobic bacteria. The OP hasn't stated what his primary enclosure for the fish is or if there is even substrate or how deep it is, etc. etc. We unfortunately are going on very little info from the OP. One thing is clear though, having ammonia spikes means the setup leaves much to be desired. It strongly reminds me of my beginning years in the habit of keeping large fish in large quantities. The OP primary nitrification is not working long term, not stable, that's what killing his fish, why are we squaring swords over nitrates and anaerobes? It's strange to me at least. Let's focus on that truck flying 100 MPH into our face and forget for a bit about the potholes...
Far, far most usually, people don't test nitrates right, or confuse them with nitrites, and reporting zero nitrates is a common red flag that something is wrong, either with the system or with the keeper and how they test.
As a mod, I please, ask you to stay civil and choose words more carefully, address others the way you wanna be addressed. Think of yet others too - we don't want to waste our times reading an exchange of witty unpleasanties...