RO is perfectly valid and safe to use, and is better than their tap water source. They know what they're doing with RO, saltwater is not the same as freshwater and most do have to use RO to get it right with the salt for their setups.
They have experience with it, I'd have no concerns with them adjusting it to levels where a particular fish would need it. I'd be a little more wary with a novice trying it because it's a little bit extra to learn, but someone who already knows how to manage it, this isn't a difficult task to manage.
Sure, it's "easier" for most people to get fish that suit their tap water. But sometimes, especially in cases like OP's with liquid rock water and high nitrates straight from the tap, RO is actually the better option. 50ppm nitrates to start will kill a lot of fish, it's not safe. My cories would drop like flies in that amount of nitrates and so would my apistogramma. Even on tougher fish, 50ppm nitrates is a constant stresser on fish and will affect their immune system being under that constant stress. You won't have long lived fish with it. And to lower those nitrates is a lot of work, more work ironically than using RO and remineralizing the water to levels you want.
Ideally, people shouldn't chase parameters, but sometimes it's necessary. And I do believe this case it is necessary. And even if not, if someone wants to keep certain fish but doesn't have suitable water and they want to go the RO route, people shouldn't tell them off for that as long as they do it safely and properly.
They have experience with it, I'd have no concerns with them adjusting it to levels where a particular fish would need it. I'd be a little more wary with a novice trying it because it's a little bit extra to learn, but someone who already knows how to manage it, this isn't a difficult task to manage.
Sure, it's "easier" for most people to get fish that suit their tap water. But sometimes, especially in cases like OP's with liquid rock water and high nitrates straight from the tap, RO is actually the better option. 50ppm nitrates to start will kill a lot of fish, it's not safe. My cories would drop like flies in that amount of nitrates and so would my apistogramma. Even on tougher fish, 50ppm nitrates is a constant stresser on fish and will affect their immune system being under that constant stress. You won't have long lived fish with it. And to lower those nitrates is a lot of work, more work ironically than using RO and remineralizing the water to levels you want.
Ideally, people shouldn't chase parameters, but sometimes it's necessary. And I do believe this case it is necessary. And even if not, if someone wants to keep certain fish but doesn't have suitable water and they want to go the RO route, people shouldn't tell them off for that as long as they do it safely and properly.
