First, the good news is that the sword and danio still seem better and both ate heartily the blood worms I put in for them. The bad news is that pH and KH have dropped again. Measurements are 12 hours after my last post:
pH: 6.0
ammonia: 0.0
nitrite: 0.1
nitrate: 5 ppm
kH: 1 drop
gH: 7 drops
tank temp: 79* F
(I'll be honest, I'm not sure exactly what my nitrite levels are at. The indicator card shows 0 as a pale blue. 0.25 is the next color indicated and that is purple. My water is definitely not testing purple, but since it's slightly darker than the 0 marker I've been calling it 0.1. I'm using the API test kit.)
So, another water change (bigger this time, 25%), more Proper pH, and a tiny bit of pH UP... That's about all I can do at the moment, right?
Thanks, I found some great videos on YouTube of DIY sponge filters. My only concern with them is weighing them down. Maybe I'll try to work an air stone inside one?
EDIT: Oh, one more thing. I noticed the Proper pH instructions say "not to be used in aquariums with plants." The only explanation it gives in the instructions is that it's a phosphate buffer. I really don't understand the implications of that, but I do have 3 small plants in the tank. I'm thinking the worst that can happen is my plants die. I'd much rather have my plants die and stabilize my tank, so I am using it anyway.
Don't take offense but I think you maybe over complicating things. A pH between 6.0 - 8.0 is perfectly fine for keeping most fish without any ill effects. Only time you need to really concern yourself with the pH of your tank is if 1.) You see a drastic drop in pH, down to dangerous levels
i.e. 5.0 & below as this starts affecting reproduction & overall health 2.)If you are observing drastic pH swings (a swing of 1.5 or 2.0 is drastic, a swing of 0.4 or 0.8 is nothing to be alarmed about) & 3.) If you are trying to induce breeding behavior in "soft" water fish species (
e.g. discus, tetras, rasboras etc).
Fish can adapt to a wide range of pH levels, especially tank bred/raised specimens like most of the more common species in the hobby today. Ammonia however is a different story, it doesn't take long before trace amounts of ammonia begin to approach lethal levels & start killing off fish. This is why I suggested that you concern yourself more with your ammonia, nitrite, & nitrate levels rather than your pH of 6.0 (which is fine).
A lot of things can affect pH. This is why in my first post I asked if you are using well water, tap water, R/O water (reverse osmosis) etc. Tap water is usually treated with buffering agents by the water supplier (company) so the pH of tap water is usually more stable. Well water on the other hand is usually not so it's not uncommon for the pH level of well water to change as much as by 1.0 when introduced to your tank. The pH of well water can also vary greatly from season to season, in tap water it does not. What your tank water's pH level ends up being all depends on what you have in the tank. If you have driftwood in your tank, especially a newer piece that still has tannins in it, it can soften (lower pH) your water over time. If you have a lot of rocks & stone decor in your tank, that will harden (raise pH) your water over time.
Test your water before adding it to your tank. If it reads 6.8, make a note of it & proceed adding it to the tank. Don't test your water's pH immediately after adding new water, wait 1 day. The next day test your tank's water & if it reads 6.4 or 6.0 then make a note of it & wait one more day before testing again. Test again & if it reads at 6.0 & stays at 6.0 then that's fine, that's your tanks pH. Something in the tank, be it driftwood, CO2 (pressurized or DIY), peat moss (if you are using any) etc, is softening the water. Or perhaps like I suggested, the water you are using isn't being treated with a buffering agent hence the change in pH you are observing after you introduce new water into the tank.
Refrain from using pH up, pH down, "water softener" pillows, or any other chemicals as all these items do is compound your situation, not help it. They'll give you what appears to be adequate pH readings in the short term by replacing Mg++(magnesium)& Ca++(calcium)ions with Na+(salt) or PO43-(phosphates) but fail to achieve stable pH levels in the long run.
If you want to increase your water's pH from 6.0 to say 7.5 - 8.0 because you are keeping great rift lake cichlids or something along those lines & you want to get them breeding then you can achieve this by removing any wood in the tank & replacing it with stone decor/rocks & adding some crushed coral to the tank. On the flip side, if your water's pH is 8.0 & you want to get it down to 7.0 - 6.5 because you want to attempt to breed tetras, then adding driftwood to the tank & filtering your water through peat moss would achieve this. Both of these methods provide you with much more stable pH levels than using chemicals like pH up or pH down. If you are not trying to breed your fish & your pH is between 6.0 - 8.0 then it's fine, let it be. The same applies to kH & gH, unless you are trying to breed fish, don't sweat it, as most fish well do fine in varying kH & gH levels.
Like I said before, I suspect the culprit is a min-cycle or recycling of the tank due to an initially cycling the tank improperly. Keep testing your water for the next few weeks (2 1/2 - 3). If you get readings resembling the ones below then you can rest assured that your tank has cycled completely & look to start adding more stock in waves.
pH: 6.0 - 8.0
ammonia: 0.0
nitrite: 0.0
nitrate: 1 - 5 ppm
If your pH level is still really nagging you then go out & buy yourself a low/high range pH test kit that tests for ph levels below 6.0 & above 8.0. That way if you suspect that your pH might be lower than 6.0 & it's what is causing you problems then you can treat it accordingly. By using pH up & pH down you are attempting to treat a cause without a full diagnosis while altering test readings haphazardly.
)to raise pH & things like driftwood, peat moss, or diluting your water with R/O water from an R/O system to lower it. However IMHO an R/O system can be expensive & is not worth it unless you are keeping/breeding FW Discus fish or if you are keeping SW tanks with corals etc. JUST STAY AWAY from pH UP & pH DOWN, both are a bad idea. 
