Since you live in Egypt Nouran, you could even keep your betta outside in a pot or tub during the warmer months. Some floating plants such as water lettuce or duckweed (not sure what plants are available to you over there) would help keep waste down and give him somewhere to hide.
Not only would he have more room to swim around, but I'm sure he would be happy feasting on insects and mosquito larvae.
Just an idea I thought I'd put out there.
Also, while a 100 gallon tank is ideal for goldfish, as big as you can afford to go is better than nothing. If you could eventually upgrade to a 40 or 50 gallon (approx. 200 litres) they should be fine to live in there permanently so long as you stayed on top of water changes and invested in a good filter.