To cable or not to cable

Hi everyone,

My 25 gal planted tank which has been set up for about 18 months is
a mess.  The substrate is anaerobic & the plants have been growing
brown stuff on them ever since I increased the CO2 from 15 to 20 ppm.
I've been battling to keep the pH around 7 instead of 6 also!

I feel I should do something NOW.  I have laterite on hand & can
get some vermiculite easily & think I may try Jim Kelleys Build a
better substrate method but I'm wondering if that will be enough
to prevent anaerobicity again (probably the tank will only be set
up for 6-9 months before we move again).

I've been gravel vacuuming every few days but the substrate seems
to stay anerobic.  I have an engineer friend who may be able to
help wme with DIY heating cables but not for a few weeks.  Are they
worth waiting for or could I lose all my plants in the meantime?