Well
1. The tank is too small, and you need to upgrade as soon as possible, because as steve pointed out, 3 barra and 2 oscars are NOT gonna fit in that tank.
2. You really need to read up about barramundi, i can offer this website to help you, but the site just gives general infomation.
http://www.nativefish.asn.au/barramundi.html
3. Here is the average growth rate of a barramundi, now just think how long that 3ft tank is gonna support 3 barramundi and 2 oscars? the barra will quickly out grow that tank is a matter of a couple of days, since they have already out grown it now.especially when feeding them live food instead of pellets
YearLength range (mm)1
1 310-330
2 430-500
3 529-610
4 610-690
5 730-770
6 810+
4. Why did you have to rescue them? was the WHOLE lake/river drying up? y not just put them back in the area where they was water, and let them live the normal life, instead of a glass tank, if there was some left. And i dont think the whole creek would be drying up since there would be alot more fish to be "saved" then a couple of barra. Wild barra are different to aquarium barra, they need specific things like live food, a BIG space. where as aquarium barra no nothing of the outside world, and they dont require such things. but they still need a BIG space.
5. Barra will co exist with oscars, IF you have a BIG space.
6. Barras will eat whatever they can fit into there mouths, like 10cm barra can fit 5cm feeders in there mouth, so just dont go putting anything in there that are half the size of the barras, otherwise GOOOOONE.
7. If you are wanting to keep them for as long as you can, i suggest going out to buy a bigger tank asap, as you dont want your fish to be stunted, barras or oscars. something like an 180gal will hold them for about 1 year with 5 big fish in the tank.
8. Where is this creek? as im sure that the legal limit for barramundi to be taken from the wild is 58cm -120cm, 60cm -120cm(gulf of carpentaria(spelling)). As i fish constantly i no this, and its in pretty much every Australian booklet about fish and fish sizes.
9. Good filtration is a must, as they are very messy fish, and with 2 oscars in the picture, its gonna get even messier.
10. Im having a little bit of a go at you because, taking fish from the wild, when they clearly have a size limit is ILLEGAL, and even if they were just a spare of the moment, when you get them home research about them asap, get as much info as possible.
Cammo X