The larvae of most fruit beetles, chafers and rhino beetles produce pellets of frass which they tend only to eat if they are running very short of food otherwise. Old larval skins are normally eaten after the moult. For things like these, I would do a partial substrate change when about half of what is in the tub is frass. At this point i remove about 2/3 of the old substrate. With larger larvae, you can take the larvae out and seive out most of the frass. What goes through the seive goes back into the tub. To this I add fresh substrate, give it a quick stir and put back the larvae.
For things like stag beetles, where they do eat a lot of frass, I would just remove half of the old substate and add the fresh stuff directly on top, making sure the larva is down in the old stuff at the bottom of the tub.
For breeding tanks I would normally set up a tank, add the adult beetles and let them get on with things. After about 6 weeks, remove the adults from the tank and put them in a newly setup laying tank. Particularly for small species where the eggs are hard to see, I tend to leave the tank for another few weeks before checking the contents. This gives all the eggs time to hatch and the larvae are easier to find. At some point I will sort through all of the substrate and remove all of the larvae. This allows me to see how many I have and divide them up into sensible sized groups for their next containers. If I am spreading out these larvae into new containers then I just mix fresh substrate in with the old at about 50/50 ratio and fill up the new tubs with this.
