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Nitrates problem (long)

Hi, I'm new to the list, and wanted to ask about some
solutions I'm considering to address nitrate buildup
problems in my 55 gallon tank.

Here's my situation.
55-tall gallon tank with wet/dry filter (ammonia=0,
nitrites=0, nitrates continually building)
96 watts compact fluorescent lighting - 5 hours/day
40 watts fluorescent tube lighting - 10 hr/day
bioplast CO2 tablet system, Flourish plant supplements
plants - java fern, annubias, one large sword, java moss
(note: these are plants that have survived the predation
of the goldfish)
2 goldfish, (1 about 6 inches, 1 about 2 inches
both sizes including tail) fed once per day, on occasion
I let them go a day or so between feedings.
tap water- extremely soft (GH and KH are 0 out of the tap
using my test kit) so I must add general and alkaline
buffers or I get ph crashes (every time I change water
I have a couple of days of unstable ph (rapid swings
down to under 6.0).

Because of the ph problems I prefer to minimize water
changes, but the buildup of nitrates is problematic.
I currently change about 50% water once a month. Smaller
water changes still cause ph problems, and have to occur
more frequently, so I prefer one big one.

I know I have a high fish load, but I'm coming from
the fish angle... in fact I want to increase fish
load to the generally recommended goldfish stocking level
of 1 fish per 10 gallons (instead of my current level of
1 fish per 30 gallons (counting the wet/dry sump)).

Needless to say I have algae problems. Because of the
goldfish, my temperature is ~ 65-75 deg F. and many
of the rapid bunch plants that might take up the
nitrates don't survive predation. I'd be interested
in extremely fast growing rooted plants. Bunch plants
get uprooted by the fish and are difficult to replant
in the tall tank. Anacharis was growing well for a while
but I had to trim and replant them and they all died.
I grew crinums for a while but once algae grew on them
they changed from unappetizing into goldfish treats...

I'm considering two options:

1. Making a DIY coil denitrator to reduce nitrates. It
seems like this might be the best solution, but all the
information I'm finding on the web is old (e.g. 1999)
and I'd like to see what the current wisdom is before
taking action.

2. Create a Nitrate sump, a separate tank that would grow
java moss or some other plant to act as a nitrate sponge.
Don't know if this would work, or if it would remove enough
nitrates. One study showed a six inch goldfish in 12.5 gallons
of water fed twice a day built up 30 ppm of nitrate in
seven days. I don't feed that often, but I'm sure you get
the gist of my problem.

Sorry this is so long, but I wanted to provide all the
background information. Anyone have any feedback or comments?

Thanks for your help.