You need to read up on aquarium water chemistry to better understand it. But your KH test measures the amount of calcium in the water only. The GH test measures what is called total hardness, that's the amount of calcium and magnesium together.
TDS means total dissolved solids. These are molecules that are not water molecules. They can be metals, heavy metals, pesticides, herbicides, chlorine, anything at all that is in the water that is not hydrogen or oxygen.
Marine keepers need to start with water that is pure, usually RO, maybe DI.. and that's because they are adding salt mix. TDS measurements are a way of measuring the amount of salt needed. There are other ways, such as measuring the salinity, a couple of different tools to do that. But if the water they start with has some amount of solids in it, it can make it more difficult to get the right ratio of salt to water.
In fresh water tanks, TDS tends to be most often used by shrimp keepers. Sensitive shrimps, like crystal and Bee shrimps, are more easily shocked by changes in the water parameters. So keepers try to keep the TDS in new water as close to what is in the tank as they can.
Fish are also sensitive to changes in TDS. But it depends on the fish species, not all are as sensitive as others. But big changes.. as yours, which more or less doubled, can really make a difference to fish, especially fry.
Try testing your tap water after it has been in a container for 24 hours, with the dechlorinator added, and it's best if it has an airstone running in it as well.
Do the same with the spring water. Also measure TDS for both the tap and spring water.
The results should tell you if there are any major differences between your tap and spring water. Not all spring water is fantastic quality.. much of it has flouride in it, some of it, I am sure, is really tap water with a fancy label on it, though that might be hard to prove or disprove, I often wonder about what's really in those bottles.
Most tap water is safe enough for most fish, if you use dechlorinator and do regular partial water changes. Topping up only does increase the TDS levels, because as water evaporates, the solids become more concentrated, and when you add more top up water, you add more solids with it. The only way to avoid this is a proper water change.
Water changes also provide new minerals, which do get used up over time in a tank. The fish utilize them, for one thing, to regulate their internal pressure.. this is osmoregulation. And as the previous poster commented, a big change in TDS may be the real reason for the symptoms of what we used to believe was pH shock instead.
Stable water parameters are the single best thing you can do for fish, and tap water, while it's usually safe, should be tested periodically, because weather events can affect what the water treatment plant adds to the tap water at any given time of year. Heavy rains especially can mean extra chemicals being added.
Hope that helps a bit.. but you need to read up on water chemistry. It's not the easiest topic to wrap your brain around, and I'm still learning, and have plenty to learn still. But the more you know about it, the more likely you can figure out what might be harming fish.
And the single most common cause of fish death is an ammonia or nitrite spike. These can happen if a fish dies and isn't found, or if something happens to the filter bacteria, and if you don't catch it, you may never realize it even happened, but later on you will lose fish. Ammonia doesn't always kill outright, it can simply shorten the lifespan, depends on how long and how strong the exposure was. I am not saying this is the reason your fry died.. but it is the most common reason of all for fish deaths.
. I do know KH is the factor most important for buffering. Having fairly high KH in our water here, keeping conditions stable is not very difficult.
