Any tips for a first time tropical tank owner? Guppys, Neon Tetras, Khuli Loaches and Catfish?

you should get a sucker fish like a plec or bristlenose, they help keep ur tank clean saves you having to do it so often! if you have live plants the snails will eat them! they are pests in my eyes, depending on the snail i guess, i dunno i never kept snails, loaches tend to eat the snails and the snail eggs... they cleaned my tank of snails in about a week and i had a good 100 small snails roaming around my tank... i only clean my tank when it looks dirty but i only tank half the water out at the most, if you take it all out you got to cycle it again etc... to get all the levels right... not sure what could be causing your fish to die tho, thats weird... maybe get your water tested?