[APD] How do I avoid bubbles in siphons?

[Rod Gerrymander <rgerrymander at yahoo_com>]

A couple of days ago I setup my dream system.  It uses
two 20 gallon high tanks as bio-remediation for a 120
gallon planted tank.

Water is pumped via a Fluval Stage 4 from the 120 
gallon tank into a 15 watt UV steralizer (I have had 
problems over the years with fish tuberculosis) and
from there goes into the top 20 gallon tank, an algae
tank with 72 watts of compact flourescents on 24 hours
a day.

>From there it flows into the second 20 gallon tank,
also 72 watts of compact flourescents (from AHS, great
people to work with), lights on 12 hours a day, 
opposite schedule from the main tank.  This tank is
heavily planted with Vallisneria species.

Finally it flows back into main tank.  This is lit 
by 12 48 inch t8s, currently 10.5 hours per day.
Other than the trip thru the steralizer all these 
flows are by gravity via 1.25 inch ID tubes.

My problem is that there is pearling going on in these
tanks 24 hours a day somewhere and the siphons are
getting bubbles in them which tend to collect at the
highest point, with disatrous consequences if left
untreated.  I need a system that can go without 
maintenance a minimum of three weeks, and this is not
going to make it.

I tried putting 90 degree U curves in the bottom of
the siphons so that rising bubbles would not enter
them
and this was surprizingly uneffective.  With all the
oxygen plus co2 in the water I'm not sure what these
bubbles are, but they may even be rising spontaneously
inside the tubing.

Does anybody have a suggestion for keeping these
siphons flowing?

Thanks,
Rod

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