I was wondering if my tank is overstocked. Its 300 litres with;

1 tiger oscar (2")
1 gold severum (4")
4 firemouths (2")
1 syontis catfish (5")
1 six barred distichodus (1")


I dont think it is overstocked at the moment but will it be eventually?

i guess you are in the clear now but with the fish growing to full capacity, you may need an upgrade.The grown up O alone will need over 200Liters at least for himself.
Allthough now they're all still very small, you'll find out soon enough if your fish need more space.
Keep checking some secondhand sites, you may run into some nice upgrade some time before you actually need it!
I got most my tank either cheap or for free that way.

edit: Distichodus will need an XL tank once he starts to grow!!!
www.aquariumfish.net/catalog_pages/wild/distichodus.htm

Yeah I thought Id have to give up the six barred distichodus eventually as im told they grow very big. But I dont think its gown since I got it, so Im guessing its a slow grower and Ill be able to keep him for a while.

as Noddy said you will be ok for a very short time but are going to run into problems sooner or later. the six barred distichodus can grow 12-15" and needs fairly roomer tanks of 150g. also the feeding is a little particular. live grass shrimp,worms, and live plants being its diet. the oscar needs 50g+.

as with a lot of people you have purchased fish without a suitable tank to house them and have not been given the best advice at the store. think about re homing or a large tank very soon.