While I don't control snail population that I deliberately add, bladder snails are a different matter. I don't follow the cruelty line of reasoning (if that was meant to apply to bladder snails?) as any thing done with them that does not involve chemical poisonings will still kill them. If you starve them to death they are just as dead. If you remove them from the tank you need to put them somewhere. The only people that buy them do so for feeding puffers and other snail eating fish, etc etc. Plus the darn things are apparently eating fish eggs so keeping them in place means they are killing fish /shrug.
If they're eating fish eggs, the eggs probably aren't viable or weren't fertilized. I'm not saying it doesn't happen but it's extremely rare. I keep Bladder Snails with every kind of fish imaginable and haven't had an issue like that - ever. Same with other snail eggs. They don't get eaten unless they're unviable. I've never had it happen until eggs start to break down on their own. Same with shrimp - they'll chow down on fish eggs but I've never seen it unless the eggs are in poor shape.
Killing snails for the sake of killing them is inhumane and cruel, yes. (Though my point was more geared toward shrimp, it still applies here.) Just like Pond Snails, Ramshorn or any other that can reproduce in freshwater, their population can be controlled by limiting their food source. That's fairly natural in a planted aquarium and balances out over time. We can nitpick and suggest managing waste in a tank is as cruel as rounding up a ton of them from a population explosion due to overfeeding but reality? That's just not the case. One is responsible tank husbandry, one is usually an overreaction from being irresponsible in some capacity. And using snails as a food source is not irresponsible or cruel - they have natural predators.
It's up to you how to you deal with your tank. No one is telling you otherwise. I'm just telling you that it's unnecessary to use Planaria Zero in this case and is, in fact, often cruel. Kills snails,
can harm shrimp and even fish. It remains in a tank or environment for a long time - potentially the life of a tank - and most of the time is absolutely unnecessary to use. It's a permanent solution to a temporary problem. 99.99% of the time it's because someone freaks out because they don't yet understand something (but will soon) and are looking for a quick fix.
The problems that led me to using PZ were the Hydra. PZ is totally shrimp safe, which is why I chose it. But, as I don’t like pest snails, I was happy when they died off as well. I wasn’t overfeeding, the snails came in at the same time as the hydra, on a plant mat from my LFS. And if killing pest snails with Planaria Zero is cruel, what about the Hydras? Where to draw the line? Had I known ahead of the time the snails were there, I would have removed them and killed them with the clove oil method.
That's just it - Planaria Zero is not "totally shrimp safe" - it can and has been problematic for plenty of shrimp keepers. Even people here on this forum. Many people don't have problems with it. Many people do. It's still safer than Fenbendazole, however.
Snail populations don't explode unless there's an overabundance of a food source. You don't have to overfeed for there to be an overabundance of a food source.
If you just have 8-10 Hydra in a tank? Yes, it's probably unnecessary to use Planaria Zero to get rid of them. Especially when the snails you already have will eat them and they can easily be manually removed. If you have a thousand Hydra or something, that's an entirely different issue that needs to be addressed. But Hydra aren't animals in the sense that snails are - animals that many people in the hobby keep as actual pets. You can consider them "pests" but that's generally a bit misguided, as snails like that play a vital role in the planted aquarium. When you're more experienced as a planted aquarist and shrimper, you'll have greater appreciation for them and will better understand their role.
Note: Hydra are rarely killing shrimp - especially larger shrimp - unless they're present in huge numbers. Based on your other thread, symptoms seemed to suggest their deaths weren't Hydra-related. Those weren't behaviors consistent with Hydra stings but were consistent with what occurs when one receives recently imported shrimp. (Yes, even from reputable vendors - almost all of them online are imported in bulk and are not quarantined.) Hydra can't easily sting adult shrimp with thicker shells until right after they molt. That's why it's usually only soft, baby shrimp that are impacted.
