Seeing as a system is bacterial driven its not alarming to see levels fluctuate depending on the bacterial processing working within. I do agree with the information and suggestions others have provided, but additionally the constant inconsistency and fluctuations you report makes me wonder if you have a nutrient imbalance which is making reducing your p levels a real challenge. What is your no3 at? I'm curious to know if you are nitrate limited? Your system could just be extremely efficient at processing no3 as It was a pre established system, being lightly stocked, and some of the means you are employing to control nutrient levels. This could also line up with the fluctuations you are seeing and a heavy feeding a few days before you test or slack on cleanings could actually result in decreased levels of po4. Whereas in an attempt to keep po4 low you reduce and limit feeding or do an aggressive cleaning etc. on a given day which could result in an increased level of po4. No3 must be present to reduce po4, bacterial speaking. Limited N to process, limited P being processed. No extra N to process, no more P being processed. But If you do have elevated nitrate levels then the above should not be an issue and you should continue to aggressively go after p just as you are and if you follow the advice others have given you will slowly get levels where you want them. If you are nitrate limited then you may have to make additional changes elsewhere along with what was suggested by others.

Thanks for your suggestions, guys.

@ Masonjames: you are right; I am nitrate limited. I have algae growing like crazy in my fuge and I am started running biopellets in the last couple of months. I only have about two inches of biopellets that I started a couple of months ago. I haven't checked my nitrates in a couple of minths, but every time I do, they're undetectable.

What ither changes do you suggest? I am feeling somewhat squeamish about ading nitrates to my system...

Well obviously I can not say with 100% certainty that this is an issue for you that needs correcting, but if you are in fact regularly getting undetectable nitrate levels then it could be very plausible that this is in fact hindering you in your attempt to get phosphate levels under control. It's almost like your system would be working against you and not with you. Not quite that drastic but gives you an idea. Most can have very simple success combating this imbalance by employing almost any means of phosphate reduction and I am sure most would not even realize this imbalance as they simply see phosphate levels and remove them not understanding the relationship they share on a bacterial level. But for others for whatever reason(s) can struggle trying to pick up the phosphate slack and a fortune spent on gfo s and the like just doesn't seem to cut it. But most don't seem to find themselves in this predicament anyways so... You may just happen to have a system that is far to efficient at processing what could be limited amounts of no3 to begin with. Raising no3 does seem very contradictory though so I get the unsettling feeling but I suggest you do some research first and come to your own conclusion whether you think it's an issue for you or not. Probably the most simple and rewarding way of trying to increase if you determine you want to is just simply start by slowly increasing your bio load. Yay more fish! If you don't have success with that you may need to start looking at your means of nutrient control and husbandry practices and yes contradictory again but you may need to slow it down some. Keep in mind you would be after detectable levels, not excessive levels. And like everything else, going slow is the name of the game.

if you need to up your NO3 then amino acids will do it,potassium nitrate and sodium nitrate will too. also you can overfeed as well up your bioload which are some of the more traditional ways;)

One thing that may be part of the swings could simply be poor testing practices little things can easily swing your results a long ways. If you get a funny number always rerun the test. One thing I've found with po4 test is it's very hard to get all the reagent out of the packet. Also entrained bubbles from shakeing the vials will really mess with the result. That's where I'd start and if it's still occurring then I'd look into making changes. If your tank is doing well other wise I'd be tempted just to ignore the numbers and leave it be.

Finally got my salifert test kit. Tested my water and my phosphate is 1.00 and nitrate 100, no wonder why I have slow growth and hair algae everywhere. I'm gonna do another water change and may run a biopellet or carbon dosing