Warm white bulbs for a planted tank...I had a really bad outbreak of BBA that I attribute mainly to them. I switched them out with aquarilux bulbs. They are cheap, provide proper spectrum, and can be found at most hardware stores.
Totally useless Red Sea PO4 and Fe test kits. I never got a decent reading from either kit despite having reference solutions that were in the middle of the kits' ranges where they should be most accurate. Swapping for new kits didn't change anything.
I dont remember the name of this (supposedly algae eating) fish. It's huge and it's mouth is formed to suck on things. I bought it at an lfs(tong's) for $24 bucks with a two day guarantee. It didnt' even eat a single bit of algae(any type) and died on the beginning of the third day.
Man, I really like my Fluval canisters... Never had a problem. Are you guys playing football with them?!
Anyway-- worst thing I ever bought (besides a 30" aquarium) was a product called Aquarium Plant Vital which was upposed to replace CO2. This was WAY before Excel which I hear works well. I think it was a Mark Weiss product...Snake oil, it was totally worthless, IMO.
Emperor 400 ??? I love my bio-wheel, I think they work well besides the priming problem, but only good for fish tanks.
The worst filter I brought was the Whisper 3.
Besides that the JBJ algae magnet and the JBJ 2 way powerhead is also on top of my worst list and of cost the plastic plants.
when I first started keeping a tank I thought it would help with my liquid rock. I should've invested in an ro unit instead. I ended up tossing it when we moved. It was a pain and I had to change the inserts constantly. thanksfully I have wonderfully soft water now. Now all I have to do is add baking soda to keep the kh up!
My stupidest purchase was a canister filter that had a clip missing..... being a canister virgin, i had no idea that it would leak A TON of water into my house.....fellas if you are going to buy a canister filter used, make sure it has all the clips!!!! the dishonest seller might not even warn you about it!!!!