Are you mixing your makeup water correctly? When you have mixed and aerated, it should be around 8.1-8.2.
7.9 isnt horribile, by the way.
More about that later...
What is your alk level? It is a dkh or a mq/l measurement. This is important.
Your Sg is too low...bring that up to around 1.025.
Again...sounds like your change out water isnt being mixed properly. This will also account for low pH.
For example, instant ocean says to mix 1/2 cup per gallon which will give you a 1.022. This isnt good for reef tanks, however. I also noticed at this mixture, my pH ends up being low too- around 8.0
So I add about a 1/3 cup to 1/2 cup more while it is mixing to bring it up to the values I want. 1.025 and pH of 8.1-8.2
Next, I think you add way too much chemical stuff to your tank. You dont need all of that crap. Your salt mix contains pretty much everything you need. Proper water changes will keep all of the trace elements in check...what salt mix are you using?
When you learn what your buffer requirements are, all you really need is alk and calcium buffers.
Here is a water change direction for you, compiled by all I have learned from bang guy and some other people on this and other boards. Since I have been properly mixing and adding, tank is much better off and stable.
You will need:
-1 cheap powerhead...like a rio 800 or a smaller maxi jet
-1 cheap heater around 100-150 watt
-1 cheap air pump and stone (somewhat optional)
-1 refractometer if you can...(hydrometers should be outlawed)
-5 gal or more bucket for the mixing( you have a 90 gal tank, you may want to look for something in the 10 gal range), another bucket at 5 gal to make it easier to dump in the tank
-salt mix
-good source water-no tap water unless its been tested
-a notepad to track all of your tests
1. fill bucket of any size about half with the water, then add appropriate salt for that amount ( i fill my 5 gal about 3gal worth, then with the instant ocean, I add 1.5 cups of salt) while stirring very well. (Salinity at the rate IO gives you will be around 1.022, thats too low, but more later about that)
Only add the salt to the water, never add water to the salt.
2. Finish adding your water to the bucket, then add appropriate salt again while mixing. At the rate IO gives you (1/2 cup per gallon) your salinity wil be around 1.022. This is when I add about 1/2 cup more to the water in addition to what I've added, and in a 5 gal scenario, it comes out to 1.025, which is about where you want to be.
3. Add powerhead to circulate and airstone to oxygenate (airstone is optional, i like it though)
4. cover and let circulate for around 24 hrs. A couple hours before you will use the replacement water, add the heater. Test salinity after water gets to targeted temperature, which for most is around 80. But acceptable range is around 77-82 give or take, varying opinions from many people.
5. Test salinity. If this is good, I suggest testing this water for ph and alkalinity and calcium and magnesium. This way you know the shortcomings of the mix. DO NOT ADD ANYHING TO THE REPLACEMENT WATER TO ADJUST except salt. It may cause a preciptaion event, which i learned the hard way one day If salinity is too high or too low, either add a little more salt or just unsalted water to adjust. If you must do this, let sit for a little while longer and circulate. Test salinity again.
6. Ready for the change? What I do, is I dump half of the 5gal into another 5 gal bucket, just to make it easier to handle the weight while pouring in. So that's why I have 2. And if you ave purchased a 10 gal mixing bucket or bigger, you could siphon out the water into the 5 gal bucket. You could also use the powerhead (thanks knowse) to pump the water into your tank instead of dumping it in.
7. Drain appropriate amount of water from your tank. Replace with new water.
8. Wait a few hours or so, then test your tank water. Given what you may know is lacking in your replacement water, plus the test on your tank water after replacement, this will give you a good idea what is needed.
Now you can add your b-ionic alk and calcium if needed and any other additives to the tank. Best to wait though to add anythig about 24 hrs and test then. But always test tank water before you add anything, and anything you add, do it slowly. Better to adjust over time than to try and quick fix things.
