Another question, why does there have to be constant heat for 320ppm? Why can't heat be added afterward like with 20ppm?
Silver has a solubility limit. At room temp, it's about 20ppm. If you keep generating silver oxide at the same temperature the water basically says, enough is enough, won't hold any more and the excess will just precipitate out and ruin your batch.
You can raise this limit by heating the water. I believe you can get it up to about 40ppm as a maximum with just heat alone. But at higher ppm you need to get the silver oxide reduced to nanoparticles more quickly than the silver oxide is being produced by your electrolysis.
Think of it as bailing out a boat with a hole in it. All the time you keep the water "empty" enough of silver oxide, you've got room to add more. The constant heat keeps the solubility limit higher and because you've already added the reducing agent, as soon as it makes contact with the newly present silver oxide, it turns it straight to silver particles, freeing up space in the cell for more silver oxide !
Ha! We overlapped (scratch that, you plain beat me to it Kephra!)
As Kephra says though, Your gelatine, needs to be properly dissolved and the heat ensures this is the case the whole time
Simples !!