I have bought a second hand tank and it needs cleaning inside and out. Will it be ok to use chemicals or should i just use water? Obviously chemicals will need a very thorough rinse with fresh water, but can they leave residue behind which will harm fish afterwards? Is there anything I can use which is safe?
Many people use a diluted bleach solution to scrub it and then let the tank run with dechlorinated water in it for a while, then drain it. I'm not to sure of the ratio of water to bleach but I'm sure others will chime in quickly.
Are there calcium deposits? You can try using a razor scraper to get those off - if it is a glass tank. If it is acrylic they sell special razors for acrylics that do not scratch the surface. Hope it helps!
What size tank you pick up? What are your plans for it?