Personally I'm not a fan of fish tanks for reptiles. They're great for fish, but not the best for lizards. They lose heat too quickly and the tops make a perfect dessication chamber. Great for jerky, but bad for long term care. That said, here are a few things I'd recommend to make the next four months easier on the both of you:

1: Careful on the basking spot. I'd lower it to something between 115 and 130 degrees F at the hottest spot.

2: Dirt is fine. I use the junk from the fields for my monitors as well.

3: Careful on the rocks. If they are not glued into place you may want to consider doing so. Your little one will tear up its cage and if the rocks are able to be knocked loose, you risk the chance of your new pet hurting itself.

4: It is normal for it to hide. Lizards that small don't do so well when they hang out in the open next to bigger animals.

5: If the lid is screen you may want to cover 1/3 to 1/2 the lid with Saran Wrap to keep the cage from drying out to fast. If you do this airflow will be reduced, humidity will rise a bit, and you'll need to be sure your lights aren't getting it too hot in there.

6: Careful on the diet. People seem to be a bit overzealous when keeping monitors from the African clade. If you overfeed it you'll have a "tame" lizard that is too fat to move and will keel over earlier than you probably want. Most pet African monitors are so over weight their bellies drag on the ground. That is a bad thing.