Mr Clark:
Do check your pumps to ensure that they work correctly with a square wave driving them. Whilest the transformer in the linked circuit (if it is an iron core device) will round the waveform some, the output still won't look like a sine wave.

The sealed aquarium pumps that I have seen rotate the impeller via an induction process and may very well not work well at all with a non sine wave drive.

Most low cost inverters that work on square wave principals are limited to driving loads that are largely resistive like light bulbs or for universal electric motors of the AC/DC types used in cheap electric drills and jig saws and so forth.

Michael Karas