To get Ph down can I use..?

Acid as well as quick ph changes are potentially deadly to your fish. I don't know what ph level you have though if it's 8.2 get some Lake Tanganykan fish. If it's 7.8 -7.4 get some Lake Malawi or Central American fish. The best thing you can do is not adjust the PH of your water with any form of chemical. Get some fish that will happily live in your aquarium and that way you can keep up on regular water changes which will provide a long healthy happy life. When you start messing with the ph eventually you forget or change the water too fast and kill everyone. If you absolutely have to raise or breed fish that can't live in your ph then you should buy a reverse osmosis system. Test the water before it goes through the system, test it after it exits your system and determine if you need to mix this and at what level to get the water hardness and ph you are looking for. If you need to lower the ph after you've removed the hardness then use canadian peat moss though boil and test this before begining because you want to know what acid level the batch you have is at to determine how much you want to filter through your aquarium. If you go this route you can have the ro unit put water into a 55 gallon drum and have some water movement in here that will filter through the peat moss. This way you'll have a constant supply of water at the level necessary for water changes. As mentioned from the begining, life is much easier to get fish that can live in your water. I ume based on your ph comments that you water is very hard and that's great for a wide variety of fish and I'm sure you'll find some you would love to have in your aquarium. Enjoy.