in my experience with the "valve splitters" it seems like the first 2 or maybe 3 tubes coming off have the best pressure and the last 1 or 2 have little to nothing coming out. this may have been from a faulty or crappy "valve splitter" or more likely not a strong enough pump but i just ended up getting fed up with it and giving it to my dad
( its sitting in a box full of fish supplies because he doesnt use it either )
but like i said, it could have been a faulty guy or not so great pump. i know they make some "super power heavy duty" pumps but these are expensive.
i personally am a fan of a free RENA pump i got recently and i think every air pump i buy in the future will be RENA (alittle more expensive though)
im sure this could be very controversal but IMPO air pumps are more required to disrupt service tension to let "bad gases" escape more so then putting "good gases" into the water, but regardless of what its doing (either letting em out or putting good guys in) its doing something good.
i am looking through www.thatfishplace.com's magazine right now and they have a WHISPER DEEP WATER AIR PUMP by TETRA which is rated for 150gal. duel outlet and is $58.99 which im sure you could make work.
there also is WHISPER AIR PUMPS(by whisper) for 10gal single outlet pumps for $6.29 a pop (x10=$62.90, which i think is rediculous considering you will also need x10 power outlets or x5=31.45 which is alittle more realistic but again will take up x5 outlets which is like a full power strip bar)
i also see Coralife Super Luft Pump (2.9psi,22W=$63.99 and 3.9psi,66W=$72.89) but these have multiple out valves however from the picture they appear large(they dont have the dementions in the magazine) and prob. going to be alittle noisy.
i hope something from here was helpful, goodluck!