Help My fish are dying

HI
I have a 30 gallon tank. I bought a fish from a guy and since then my
tank got all screwed. I did a water change but all my fish are dead
except for the two clwon fish. I tested the water and here are the
results.
Ph 7.7
Ammonia 0.3
KH 300mg/l
Nitrate 0mg
What do I need to do to keep my two clown fish from dying and bring my
tank back to normal. The clowns are lying on the bottom no moving
alot.

If someone can give me some guidance, it would be greatly appreciated.

G