Okay, simple facts - 125 gallon tank, 175 lbs live rock, CC substrate, corals withering and not doing well, Ammonia 0, nitrite 0, nitrate at 200, pH at 7.8. specific gravity at 1.020, think I saw temp was low also, and you are using a wet/dry filter.
Get ready to learn something!
You cannot in all likely hood fix everything at once without loosing livestock. So let's take it one step at a time as cost effectively as possible. First mix up 20 gallons of saltwater, mix, let stand 24 hrs, it would be better if it were heated and kept moving with a PH.
pH, temp, nitrates and specific gravity all need help so try this.
Shiut down return pump. Open up wet/dry filter and remove your bioballs,
clean the sump out (filters, sponges, equipment, everything gets cleaned and checked to see it's working). Now find enough live rock rubble to place some in wet/dry where bioballs were. Set heaters to 79 -80 range.
Notice that was plural, using 2 heaters instead of one large one is the prefered method. Now gravel vac your entire CC substrate surface (Your rock will work do breakdown Am and reseed CC. Make sure your new mixed water is at 1.024 SG. Put it in.
Your nitrate level of 200 is way too high. Skimmer overflowing into tank, bioball production, CC gathering detritus, etc all increase this. The live rock rubble will break Am to NI and NI to NA just as the bioballs, but it will
also break down the nitrates that the bioballs will not.
Next week you will want to do another 20 gallon water change with water mixed up to 1.025. This way you are slowly changing the tank up to desired levels. You will need to keep doing weekly water changes until your SG is at 1.024 or 1.025 in the whole tank. Now once it is at that level with a temp of 79 - 80 we can work on your other parameters.
If once your entire system is running at 1.024 - 1.025 your pH should be higher than the 7.8 you are currently at. If it is still below 8.3 then we can add (Superbuffer dKh from Kent) and raise it the last bit. When you are trying to raise the ALk to raise pH you need a balanced Calcium to Alk level. Both tests are easily done with Hagen Test kits. Shoot for Ca level of 440 and Alk level of 180 - 200.
Once you have changed enough water to slowly reduce nitrates, bring up your SG, raise your pH, get your temp into 79 - 80 range, raise both Cal and Alk to correct levels, begin emptying skimmate from skimmer, and doing regular maintainance weekly and monthly - your tank will run better.
Now when you are able to afford it, start buying aragonite dry reef sand.
It is available if you search much sheaper than live sand. Once you have enough for a sandbrd depth of over 4 inches plan a day to replace all CC.
If you try to do it in sections you will disturb the lower levels of bacteria in the CC that should not be! Trust me on this. Once that is done your lighting is in question. If your current fixture is just 2 - 175 watt MH you need to think about adding some more light. But that will be dependant upon what corals you want.
Seriously try this plan, everyone trys to help, but from just your posts within this thread I picked up what I believe you need to do. As always read as much as you can, put your books by the bedstand if you have to and reread until you know that book by heart, then get another!
Just a thought.
