If you like what the coral is doing keep it around. If you don't have much gravel Nika, use the coral in that. One can also pack a box filter or even a section of a power or cannister filter with coral rubble.

Because of the ability of coral to cut, don't keep Corys with it. That may be a reason not to keep clumsy guppies with it. :0

It may be better contained within a nylon or synthetic mesh bag. Quantity and contact with the fishes can roughly be controlled that way.

For a quick fix if the pH is still below 7, dump in a level quarter teaspoon of baking soda in a ten-gallon tank. I use so much of it as a cleaning agent, and to neutralize the vinegar which I used to neutralize the bleach on my hands while cleaning tanks that we buy a big 12-pound bag of the stuff from Sam's Club for less than a dollar a pound. For everyday use we still put it into one of those little old boxes. There is an appropriately sized scoop in the big bag which my "baking soda fairy" graciously uses to refill the box.

It also is useful for scrubbing down before working with tanks. Not the bactericide that any soap is, but who cares if trace quantities of baking soda gets in the guppy water? :)

If you are struggling so much with your tap water, maybe you ought to try adding a tiny measurable amount of baking soda per gallon when you draw water to sit for a couple of days. Put it in the container first when you put water conditioner in and run the water. That sounds a little crazy but that may be what you have to do add the least expensive buffer.

I also use a couple of teaspoons of baking soda while soaking out the coffee pot and the brewing basket. I'm fond of my coffee but don't need it terrifically acid from the residue on the equipment.

We used to just run a pot of water and a dollop of bleach (a much more serious base) through the office perk at work every now and then (mostly then). It ceased to taste like it had been brewed in someone's boot. ;)

With all of our uses for baking soda it still takes a few years to go through a 12 Lb bag. ;)

Your experience with tap water is very different from mine. With our liquid rock from the municipal wells (up to 400 ppm hardness, not counting the other stuff), I'm cutting that tap water with 50% RO water even for guppies and other livebearers!

Please continue to let us know what is happening with your water. By the way, (guessing that you live in the US) give your water department a call someday and ask if you may have a copy of whatever  report they give the EPA. They are supposed to submit periodical reports and should have copies for local citizens. It might be useful to know what is in there.

I need to do that again for Park Forest, Illinois too. Thank you for the reminder. :)

When I finally found the report on the field studies for that Suriname study mentioned a few days ago at http://portal.conservation.org/portal/server.pt/gateway/PTARGS_0_122818_111125_0_0_18/43_RAP_Bulleti n.pdf
I was astonished at the relatively "high" pH reading for that region, where pH readings may go as low as 5.5 to even 3.1 !

In that undisturbed area mostly under the rainforest canopy, I expected the water with very low mineral content (as indicated by the conductivity readings) and relatively cool temperatures for being near the equator at those altitudes. What I hadn't expected were the pH readings of right around 7! I guess that happens in some places undisturbed by mining and "development."

So how can readings stay at so "high" a pH in such pure water? 1. The relatively lush algae and other plants sucked up the the ammonia given off by the fish and 2 "mother nature" does a lot of water changes. ;)