Try another test kit of NO3 (Nitrate)and what kind of test kit are you using, and do not defend on water change to boost up your other low minerals use a supplement, also how big/small your tank is?, are you mixing/ RODI water or NSW.
I have a Melanurus wrasses and it eradicated my flatworm and controlled the bristle worm population,for me its the most effective and colorful wrasses in cleaning up pesky worm
Small Clam will demand more calcium and light water change will not be enough, I would suggest dozing Calcium, Alkalinity & Magnesium to maintain the water parameter in order, also feed or seed your tank with phytoplankton the clams will love.
Sorry was not able to response to busy with Honey do this project LOL
but here are some update pictures
ME LOL
will add more in the next few days thanks for watching
Stand description
1" X 1" square steel
Floor to tank 44" height total height 84" X 50" X 50"
Tank description 48" X 48" X 20" X 3/4" Acrylic
Sump 2 X 60 gallon plumb together with fuge
2 X ASM G3 skimmer
Aqua logic delta star 1/3hp chiller
55 watts UV filter
Reeflo Dart 3600 gph return
Light spec.
2 X 250watts MH PFO with phoenix 14K
4 X 48" T5 UVL 454
2 X 112/ea X 1watt/ea LED fixture
Cycling stage
Note; Not all parts mentioned are in yet at that stage
OHHH!! LOL gotha, It's just a teaser coz we can't ship all the way there, but when ever anyone visit San Diego we will be happy to accomodate even just a visit.
Here are some close up pictures the last week before we went on vacation hope you don't mind that i rederect you to our local hobby site
http://www.seareef.com/SD/showthread.php?3255-New-Photos!-Check-these-babies-out!!!