Please do not use quick cure again, you need to identify the disease and use an apropriate med for it. In the case of quick cure, treating the tank will only make things worse; formalin merely posions the fish.
Your goldfish and plecuastomos died becuase they are both very messy fish wich will require a tank of atleast 280g in the end, 75g to start, actually two of them, goldfish are coldwater while plecos are tropical.
You do know you need to let spring water sit for a day before using it right? They add ozone to bottled water for the same reason they add chlorine or cloramines (and in some cases, such as my town, ozone) to tap water,to kill bacteria, and they can have the same effect on fishes and invertebrates (snails, shrimp, ect.).
Ay this point in time, your tank is going through the nitrogen cycle, and amonia and nitrite will be reaching toxic levels, so you need to do daily water changes of about 50% to keep those levels down and keep your guppy and snail alive.
Your goldfish and plecuastomos died becuase they are both very messy fish wich will require a tank of atleast 280g in the end, 75g to start, actually two of them, goldfish are coldwater while plecos are tropical.
You do know you need to let spring water sit for a day before using it right? They add ozone to bottled water for the same reason they add chlorine or cloramines (and in some cases, such as my town, ozone) to tap water,to kill bacteria, and they can have the same effect on fishes and invertebrates (snails, shrimp, ect.).
Ay this point in time, your tank is going through the nitrogen cycle, and amonia and nitrite will be reaching toxic levels, so you need to do daily water changes of about 50% to keep those levels down and keep your guppy and snail alive.