Is my Red Sea phosphate test kit reading high

Skydvr · Jan 11, 2015

My tank has been at .08ppm since the tank stabilized after cycling. I am quite happy with this level as I mostly have soft corals and .08ppm is not excessively high for SPS. This is using the Red Sea Pro phosphate kit that uses the color wheel and a second sample of the tank water to eliminate errors in color comparison due to dissolved organics causing a yellowing of the water.

I decided to test my new water to see what the difference was and how quickly phosphate was coming back up after the water change (100% on a pico). The new water tested .08ppm. I ran the test on my RO (Zero Water), which is at 3TDS, and got .08ppm as well. I checked it under two different brand daylight bulbs (6400K and 6700K) as well as under halogen. All the same.

Question 1: Does the Red Sea phosphate work on freshwater? The .08 on the RO could be coincidental.
Question 2: Is 3TDS anywhere close enough to register phosphate at .08ppm? My guy instinct is no, but I am not sure on the relationship between TDS and ion concentration or of that relationship to ppm of phosphate. It would also seem unlikely that all of the TDS was from phosphate only.
Question 3: Is the Red Sea kit known for being off at the lower end of the scale?
Question 4: Should I consider .08 the minimum detectable and shift higher readings down based on RO reading .08ppm, or just take all readings as they are with .08 being the minimum detectable? I'm more concerned about stability than the actual values (barring any high readings). The kit did register higher values when I first got the rock and had some die-off (separate container and a couple weeks with water changes brought everything down to current values).

Randy Holmes-Farley · Jan 11, 2015

I'd replace the DI as soon as it rises to 1 ppm TDS (assuming that is an inline meter).

When a DI first depletes, the product water can have more phosphate than the incoming tap water.

That said, 0.08 ppm in the water used for top off and water changes is still very small compared to food additions of phosphate, so it is not necessarily a huge problem.

tonizzy22 · Jan 11, 2015

The red sea phosphate pro is garbage! I always got .08 with it too. I have a Hanna Phosphate Checker and like it very much but if I had to do it again I would get thePhosphorus Checker instead. It is suppose to be more accurate.

pfoxgrover · Jan 12, 2015

I have also found that the red sea pro phosphate test is awful. Mine always read 0.08. I got a new improved reagent with a new different color wheel from red sea and now it reads lower but never matches the Hanna Phosphorous checker.
Also I find the Hanna Phosphorous checker Extremely difficult to get consistent results.

tonizzy22 · Jan 12, 2015

pfoxgrover said:
I have also found that the red sea pro phosphate test is awful. Mine always read 0.08. I got a new improved reagent with a new different color wheel from red sea and now it reads lower but never matches the Hanna Phosphorous checker.
Also I find the Hanna Phosphorous checker Extremely difficult to get consistent results.

Thanks for heads up. Guess I will keep the phosphate checker then.

Skydvr · Jan 12, 2015

I'll have to contact Red Sea about that one then. I'll find out if the phosphate test is valid on freshwater as well. I know some tests are valid only on saltwater, or they have a different color scale.

I normally change it out when I start reading 1 or 2. Although I never remember reading anything below .08, so I'm inclined to believe it is the kit. I will have to check my log to verify that though.

I've considered the Hanna meters, but with all the complaints of the timing out issues and people contacting Hanna about it with no changes made to the time out feature, I'm not really inclined to give them my money.
Maybe I'll consider creating my own for my senior design project.

Randy Holmes-Farley · Jan 12, 2015

FWIW, I've not heard of a phosphate kit that could only read salt water. The chemistry generally doesn't require seawater.

Skydvr · Jan 13, 2015

Okay, thank you. I know I've seen one or two tests labeled strictly for saltwater or fresh water.

Randy Holmes-Farley · Jan 14, 2015

Yes, some kits do have such requirements.

Good luck.

Electrobes · Jan 14, 2015

Randy, I know you broached this topic before, but are there any test kits or testers worth using when it comes to phosphate? If I remember correctly, the answer was pretty much a "no". But I can't remember and a search is produced opinion bombs everywhere about which was better than others.

GIT RITE · Jan 17, 2015

Electrobes said:
Randy, I know you broached this topic before, but are there any test kits or testers worth using when it comes to phosphate? If I remember correctly, the answer was pretty much a "no". But I can't remember and a search is produced opinion bombs everywhere about which was better than others.

Mr. Randy, I would highly value your opinion on this as well. At a minimum please tell us how do you check for phosphorus/phosphate concentrations. You could always PM your answers if you don't want it to be public knowledge.

Randy Holmes-Farley · Jan 17, 2015

I do not generally check phosphate, but rather monitor nutrients by the look of the tank (such as algae growing on the glass). I've used and like the Hach PO-19 kit, but it cannot read below 0.03 ppm. I recently got a Triton test which also gave phosphate. If I were to need frequent testing these days, I'd probably get a Hanna meter.