I've injected cakes with everything, and plain water seems to be all that is necessary. Unless you build a non-nutritious resevoir in the middle of the cake, you can't inject more than a cc or 2. I've made cakes with middles of moistened water absorbing crystals (found in nursery stores, Agrosoak is a brand at Lowe's). The non-nutritios center doesn't get filled tightly with mycelium, and the BRF doesn't glue it together. You can inject a good deal of water, but the perlite is much easier and more effective. Resevoirs can also be made in the middle of the cake from pure moist verm, again perlite method is much better. The MMGG covers these resevoirs in the optimizations section.

I've gotten up to 5 flushes from the perlite method with Ecuador cakes.

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-From a registered Mad Scientist

"From a certain point of view"
-Jedi Master Obiwan Kenobi (also a Mad Scientist tm)