Pretty cool game! Once you get the rules it's quite fun and challenging (in the hard mode). The music tracks are fantastic! I wish the level track would change a little over time during a play session. I have limited knowledge in music/sound design, but maybe you could add some additional layers to the loop-track, once the player reach certain scores. That could increase the tension. The button sound is a little bit too loud IMO.
I think the rules are simple enough, but if you would display them somehow next to the playfield for the first few rounds, it could help, I guess. Or at least they could be accessible from the pause menu.
The background doesn't feel distracting for me. It looks fancy...I like it :)
Like others already mentioned, you could try to bring more varied "siphons" in the game. Maybe that allows for even more strategic thinking.