No casualties this morning thank god. Really expected half the fish to be ill today but looking ok.
Am still feeling pretty stupid though! <a href="https://www.plantedtank.net/forums/images/smilie/icon_sad.gif" border="0" alt="" title="Sad" >:-)</a>
A few notes.
1) it's extremely unlikely that a pH change will kill your fish, so take a deep breath. Panicking causes more problems than stopping to carefully consider.
2) don't feel stupid. This is a complex hobby with a lot of unseen elements. That's why this forum is active, everyone needs help sometimes to puzzle things out.
3) as much as everyone wants to recommend huge water changes, it's almost always better to do smaller ones more frequently than to do 50% changes. This can actually kill your fish if your new water is hugely different from your old water. (i.e tap has 0 nitrate and tank has 160 nitrate) Smaller changes are fine, even stimulating for fish, but very large, very sudden ones aren't good, especially if it means a huge change in nitrate levels. There are a few instances where large WC's are warranted, but they shouldn't be considered a panacea, or cure for every and any problem.
4) if your pH has changed, it has changed for a reason. Any change in your weekly fish tank habbits could be the cause. Keep in mind that your tap may have a different pH when it's fresh, than when it's been sitting for 24 hours, so that could be a part of what you're reading. It could also be an error on your part with the test, or your test strips or chemicals could be quite old. Either way, figuring out WHY it's changed is just as important as trying to correct the problem. WC's might bring your pH back to it's usual level, but if you don't know WHY it dropped, it could very well drop again and your back to square one.
5) If you have a GH and KH test kit, try checking those. Water hardness can raise, and also stabilize your water parameters in regards to pH. If you are adding plants to a tank with almost no mineral content you'll have two problems: your plants will do poorly as they need calcium and magnesium to live. Those minerals are usually found in most tap water at various levels, and those are the minerals that make water "hard." Secondly, if you have very low mineral content (GH& KH) plants will consume parts of that over time, and as they do, your water will become so soft (i.e. lacking in minerals) that your pH will swing much higher or lower when it is affected by any type of basic (high pH) or acidic (low pH) chemical. This could possibly play a role in your pH change recently.
AS FOR YOUR NEW TANK. (Sorry this is getting quite long)
Fish food will work, but it will be an unsightly mess and far slower to work, than to do what has been recommended: dose ammonia or buy a bacteria boosting product.
If you buy some, just add a tiny bit every day and measure your nitrogen cycle parameters like you've been doing.
If you'd rather skip the purchase element, just remove all that leftover food, and add a few snails. They can stand a bit of ammonia, just feed them sparingly while you keep a close eye on your parameters. They will provide you with a slow release of ammonia, to establish your BB (beneficial bacteria). If the ammonia levels get too high you can stop feeding for a day or two and/or do a small water change.
Also, if you have too many mystery snails, see if your pet shop will take them off your hands in exchange for store credit. Then you can buy ammonia/bb bacteria/GH or KH tests.
Lastly, if you are still reading, congrats, and sorry about the shakes. My friend has them too and it can make life complicated when they flare up.