Diatoms? in a mature tank

[gldnegle76]

My 46 gal/20 gal sump has been set up for about two years.
About a month ago I noticed what looked like brown diatoms on the sandbed.
About a week ago the color shifted to more green. They show up when the lights are on and cannot be seen when lights are off.
Here's some info:

Ammonia 0
NO2 0
NO3 ND
PO4 ND
Ca 430
Alk 6.7 dkh (low)
Sg 1.025
SWC160 Cone in sump.

I have not tested for magnesium or silicates. I use an AP tap water purifier for all water changes and top off. (I did get a reading of 10ppm TDS on last batch from TDS-3 HM Digital Meter) My tap water reads about 165ppm.
I thought at first it was silicates, but I run phosguard in a reactor and the tap water purifier is supposed to do a good job removing them as well.
I feed every other day, 5 medium sized fish(probably more than I need for that size tank) and 5-6 colonies of mushroom and zooas. Coralline algae is doing quite well. Have plenty of small hermits that are more interested in keeping live rock clean than sandbed.(CaribSea special grade reef sand, about 2-3 inches)
My LFS told me that if I get my alk/ca in balance, this diatom/algae should start to go away. I bought some EVS two part that I plan to use.

I'd like some of your thoughts.
Could an imbalance of alk/ca be causing this?
Do I really need to invest in ro/di? (probably stupid question)
Do I need to order a magnesium test kit just to eliminate that as a possibility?
I did put a JBJ true temp heater(with plastic cover) in the sump about two months ago. I've heard new plastic items can cause silicate spike.

Thanks for any advise.

[HighlandReefer]

It would help to know exactly what an AP tap water purifier consists of? Does this filter include a RO membrane or DI resins or is it just a carbon/sediment filter?

[gldnegle76]

Hope this link works. It's a single tube with carbon below and di resins above.

http://www.marinedepot.com/Aquarium_Pharmaceuticals_Tap_Water_Filter_DI_Only_Tap_Water_Filter_Systems-Aquarium_Pharmaceuticals_API_MARS_Fishcare-AP4111-FIRODI-vi.html

[HighlandReefer]

When the DI resins become saturated, a lot of silicates & ammonia can be released quickly since they are held loosely by these resins. So when saturation of this filter occurs it can result in an increase in diatoms, algae & perhaps cyano. As the diatoms take up the released silicates & are removed, then the algae or cyano can take over to reduce the remaining nutrients which is what you may have going on. This will depend on the amount of silicates and chlorine or chloramines in your tap water, with higher amounts depleting your DI resins much quicker. ;)

Therefore with a small amount of DI resins & no RO membrane, you will most likely have to change their filter cartrige fairly frequently, as soon as the TDS starts to climb.

It will be more cost effect to buy a good RODI where you can simply replace the DI resins only rather then paying their price for frequent new cartridges. ;)

[gldnegle76]

Thanks Cliff.
I was planning on getting a good ro/di soon.
Do you think the alk/ca imbalance can be ruled out?

[HighlandReefer]

The idea of a balance between alk levels and calcium levels does come up quite a bit since notes are added in the Chemistry Calculator.

There is no ideal balance for calcium and alk levels other then that they both are maintained between the recommend guidelines. :)

Alk: 7-11 dKH
Calcium 380-450 ppm

FWIW, you can pick your levels and it will make little difference unless you want greater sps growth, which then you would want to maintain both alk and calcium at the higher ends of the recommended ranges. There is scientific evidence that higher alk and calcium levels produce greater skeletal growth in sps corals. ;)

[gldnegle76]

What I've taken from this is that this is probably an issue of water filtration and I should invest in a good ro/di. The tap water filter is obviously not catching all the contaminants.
Any other possibilities?

[HighlandReefer]

If you are not running GAC, it will help to reduce dissolved organics in your water column. High levels of DOC can lead to pest issues and problems for coral.

[gldnegle76]

I've been running seachem matrix carbon and phosguard in a reactor. Was using gfo but switched to phosguard when I thought silicates were the culprit.