Backyard Aquaponics
A place to discuss aquaponics

Yes a water change will help lower the ammonia level but I think you should do it gradually... 20-30% initially... I would also suggest to change your system to continuously flooding for a few days... (ie. no timer)...

It looks like you have about 3 BYAP growbeds... BYAP recommends 20-25 fishes per growbed.... so 75 fishes in total, regardless of size.... so you were overstocked...

Also bear in mind that, it does not take much fishes to produce the require nitrates for plant growth... you can stock your system less than the recommended (75) and still have good plant growth... if the other conditions are right...

The ammonia will go down with a water change just from the dilution effect.

Reckon you will only get good results when the tin roof is replaced with clear sheeting to get the light levels up. In summer you will have to rig some shade cloth up over the roof so you keep the heat out.

100 x 2cm barra isn't overstocking really. Not until they are say 10cm, but I doubt whether 100 really small fish would actually all grow out. As they are barramundi and are graded continually when small because they have a tendancy to eat each other.
Our advice is generally 20-25 advanced fingerlings per grow bed. Sandi is perhaps limited to what is currently available due the (kind of) remote location.
Don't give up Sandi, we are here to help and support you.