I have been given a tank that is a bit old and has quite a bit of old algae and gunge on the sides it is dried on and difficult to remove. Any magic tips out there?

I know that for water stains/calcium buildup you can use a paper towel soaked in vinegar. As for the algae... no clue.

Goodluck!

i recently bought a really dirty tank that had been in storage for some time.

i put it in my bath and used the power shower on the hottest setting to remove the dirt. I also used a scouring pad and used a lot of elbow grease.

I was worried a scouring pad would scratch the glass?

You can try having the tank sit in the hot water for a few hours, and then with an algae pad remove the algae.

I would use algae destroyer and soak the tank with it. Then the algae should come off. -_-

I have used a weak solution of hydrochloric acid in the past. This is reasonably radical but if the methods mentioned by the previous posts don't work, this is the way to go. You will need to take all the safety precautions such as wearing rubber gloves, protective goggles etc.
If the tank is crusted with lime etc. you will see the acid working on the encrustations when it is applied with a brush or sponge. On completion you will need to hose down the tank thoroughly and rinse it many times.