I am new to Coldwater fishkeeping, having previously successfully kept numerous tropical tanks.
I would much appreciate some advice from anyone.

1. How many fish (or inches of fish) can be safely kept in a 24" x 12" aquarium with undergravel filtration.

2. Are there any compatability issues of types of fish I can keep.
I currently have 2x Fantails about 2.5"(overall), they are the first inhabitants of the tank. Would I be able to safely add a couple of Black Moors? Any views appreciated!

Hello dougiec and welcome to fishkeeping As its so late I will answer your post tomorrow/later but in the mean time if you read through the threads on here you will find answers already to the questions you have asked

No problesm with black moors and fantails, just make sure your tank is big enough, and watch the water quality, particarly with black moors very susceptable to bacterial infection, they just need cleaner water.
Also live food weekly is a must to maintain health.
Im not sure how many gallons your tanks holds, but you should allow 8 gal per fish for optiman growth.

I would recommend only keeping fancys together and not with single tails as I have, because the male single tails can mistake the fancys as they get bigger for females and harass them, so you will be fine with black moors and fantails, but if you keep 4 in the tank you mentioned be warned you will need to get a larger tank later as they grow, as fishy mentioned they do recommend 8 gallons per fish for growth.

well 24" X 12" = 288 sq inches, freshwater coldwater species req. 30sq inches per inch body length, so your tank of 288 divided by 30 = 9.6" of adult fish
ps dont count the tail in this equation.
hth