You could get the max volume of water in it if you filled it almost to the brim. I can see there's more capacity there that isn't being utilised at the moment, not only will it will benefit your fish by adding more space, it will slightly help in adding volume to a small pond which as Lisa rightly says fluctuate wildly in conditions, it will protect the preform too as they can degrade to some extent when exposed directly to the sun. Ponds also look much better when full.
Fresh rainwater collected in a clean container can be used to keep it topped up as it's so small. That would be better than adding tap water as that contains chemicals that encourage green water....but sometimes it's the only option ppl have, so in general tap water is treated with a specific product to neutralise the effects of the additives in it......a de-chlorinator which all good pond supply shops would stock.
You could also maybe build the soil level up a little more surrounding it and then set some flattish rocks around the edge, overhanging very slightly to disguise the edge of the preform. Some grasses planted amongst the rocks would also soften the edge and make it blend in and look more natural. They could also offer some shade which your fishes would also appreciate.
Seeing as the pond is in full sun, have you considered a solar aerator, one may work well for your situation? Don't know how much sun you get in your location, but I bet it's more than I do with my UK pond!
Just a few sugggestions and thoughts for you to consider.
