Issue with heater

[Fishkeeping411]

So my acrylic 7.5 gallon tank has a sump built into the back of it & my heater never heats the tank when it is in the sump, but as soon as I put it into display area it's fine. I have it in there right now but it makes the tank look kinda ugly. I'm not sure why this is happening. The flow rate is fine. Somebody suggested that the acrylic might have something to do with it.

Can anyone help?

[54seaweed]

thats a real easy question to answer looking at the picture you provided
between the tank and the sump first off there looks to be very minimal water exchange if any . another reason is the sump cavity is about a 1/16th of the size of the tank the heater heats that 1/16th than shuts off a heater of that size is not designed to work in a small cavity like that anyway . I suggest you find a way of hiding the heater in the display . IMO that sump design on back of the tank is a poorly designed and I wouldn't depend on it anyhow.

[Fishkeeping411]

The flow is actually up to 500 litres an hour so the flow rate shouldn't be the issue. I agree though that the sump is obviously designed poorly if this is occurring.