...So whats going on? So far Ive had strawberries, cauliflower, apples and egg shells in there and also every second day changed their dry food (I bought stuff from ebay).I dont understand why theyre not eating, wouldnt they be hungry by now?I dont want them to die

What is the volume of your main tank?
40 Gallons.
How many hermit crabs do you currently have? How big are they?
Two crabs, Chester (small) and Charlie (Small to medium).
How long have you had the hermit crab(s) that you are concerned about?
So far today is day three.
What substrate or combination of substrates are you using? How deep is it?
Shell grit, not sure how high. I have recently learnt this is bad substrate and In the process of changing this.
What do you feed your hermit crabs?
Ordered off of the Aussie ebay place for some organic stuff, supplying fresh fruit and vege
Do you use distilled, bottled, tap or other types of water?
Distilled.
If you are using a dechlorinator, what brand is it? Does it state to remove all heavy metals, chlorine and chloramines?
N/A
What brand of sea salt are you using to supply your hermit crabs with salt water?
Stuff off aussie ebay user
What heat source are you using and where is it positioned?
A light bulb and heat mat, positioned on the right side of tank.
Have you calibrated your thermometer(s) and hygrometer so that you can interpret accurate readings?
Need to buy a hygrometer, Need to buy a more accurate thermometer.
What is the air temperature range and humidity inside the tank?
Warm side is at 26C during the day but drops to 21C at night (How do I maintain heat? is it bad if it changes?)
What is the highest temperature that the substrate near the heat source reaches?
28C