The epoxy resin has two goals:
1. Protecting the fish against the background.
2. Protecting the background against the aquarium’s inhabitants.
Ad1. The cement may affect the PH value in the aquarium. The epoxy resin ensures that this is minimized.
Ad2. Without epoxy resin fish like algae eaters can scrape some cement off the background. This makes the Styrofoam visible and that’s not something we want. Also, the epoxy resin stops some of the algae growth.
Polyester resin is not usable because it has plasticizers in it. This is poisonous.