Not unless you have an 800+ gallon tank and even that might be risky.
Saltwater is a solution because you can get fresh water and some salt and mix it and then you get saltwater.
trout, salmon, angelfish, clown fish, sharks, whales, dolphins.
NO, saltwater will dilute and blend in with the fresh water in a river. The faster the current, the faster it will mix in.
In the South we call it a Suicide.
Saltwater and Freshwater mixed is called brackish water.
It is called an estuary.
Mixing brands of power steering fuel does not make a difference. Just be sure to not mix different types of fuels.
Freon should not be mixed. Certain types of freon could damage o rings in a system set up for a different type of freon.
Almost all LFS will sell saltwater by the gallon, and there are a few places online where you can buy saltwater directly from the ocean. The other option is to treat tap water with either chemicals or an RO filter and then mix your own salt.
Angelfish are slow and sedate fish they mix well with most Tetras, Rasboras, Corydoras, Dwarf cichlids etc. Just avoid the large or extremely boisterous fish that may bully or nip their fins.
You should NEVER mix different types of batteries in any device. The charging characteristics are different and one type will charge "harder" than the other.
L2tp